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PEP 448 review
by Guido van Rossum March 29, 2023

March 29, 2023

I'm back, I've re-read the PEP, and I've re-read the long thread with "(no subject)". I think Georg Brandl nailed it: """ *I like the "sequence and dict flattening" part of the PEP, mostly because itis consistent and should be easy to understand, but the comprehension syntaxenhancements seem to be bad for readability and "comprehending" what the codedoes.The call syntax part is a mixed bag on the one hand it is nice to be consistent with the extended possibilities in literals (flattening), but on the other hand there would be small but annoying inconsistencies anyways (e.g. the duplicate kwarg case above).* """ Greg Ewing followed up explaining that the inconsistency between dict flattening and call syntax is inherent in the pre-existing different rules for dicts vs. keyword args: {'a':1, 'a':2} results in {'a':2}, while f(a=1, a=2) is an error. (This form is a SyntaxError; the dynamic case f(a=1, **{'a': 1}) is a TypeError.) For me, allowing f(*a, *b) and f(**d, **e) and all the other combinations for function calls proposed by the PEP is an easy +1 -- it's a straightforward extension of the existing pattern, and anybody who knows what f(x, *a) does will understand f(x, *a, y, *b). Guessing what f(**d, **e) means shouldn't be hard either. Understanding the edge case for duplicate keys with f(**d, **e) is a little harder, but the error messages are pretty clear, and it is not a new edge case. The sequence and dict flattening syntax proposals are also clean and logical -- we already have *-unpacking on the receiving side, so allowing *x in tuple expressions reads pretty naturally (and the similarity with *a in argument lists certainly helps). From here, having [a, *x, b, *y] is also natural, and then the extension to other displays is natural: {a, *x, b, *y} and {a:1, **d, b:2, **e}. This, too, gets a +1 from me. So that leaves comprehensions. IIRC, during the development of the patch we realized that f(*x for x in xs) is sufficiently ambiguous that we decided to disallow it -- note that f(x for x in xs) is already somewhat of a special case because an argument can only be a "bare" generator expression if it is the only argument. The same reasoning doesn't apply (in that form) to list, set and dict comprehensions -- while f(x for x in xs) is identical in meaning to f((x for x in xs)), [x for x in xs] is NOT the same as [(x for x in xs)] (that's a list of one element, and the element is a generator expression). The basic premise of this part of the proposal is that if you have a few iterables, the new proposal (without comprehensions) lets you create a list or generator expression that iterates over all of them, essentially flattening them: >>> xs = [1, 2, 3] >>> ys = ['abc', 'def'] >>> zs = [99] >>> [*xs, *ys, *zs] [1, 2, 3, 'abc', 'def', 99] >>> But now suppose you have a list of iterables: >>> xss = [[1, 2, 3], ['abc', 'def'], [99]] >>> [*xss[0], *xss[1], *xss[2]] [1, 2, 3, 'abc', 'def', 99] >>> Wouldn't it be nice if you could write the latter using a comprehension? >>> xss = [[1, 2, 3], ['abc', 'def'], [99]] >>> [*xs for xs in xss] [1, 2, 3, 'abc', 'def', 99] >>> This is somewhat seductive, and the following is even nicer: the *xs position may be an expression, e.g.: >>> xss = [[1, 2, 3], ['abc', 'def'], [99]] >>> [*xs[:2] for xs in xss] [1, 2, 'abc', 'def', 99] >>> On the other hand, I had to explore the possibilities here by experimenting in the interpreter, and I discovered some odd edge cases (e.g. you can parenthesize the starred expression, but that seems a syntactic accident). All in all I am personally +0 on the comprehension part of the PEP, and I like that it provides a way to "flatten" a sequence of sequences, but I think very few people in the thread have supported this part. Therefore I would like to ask Neil to update the PEP and the patch to take out the comprehension part, so that the two "easy wins" can make it into Python 3.5 (basically, I am accepting two-thirds of the PEP :-). There is some time yet until alpha 2. I would also like code reviewers (Benjamin?) to start reviewing the patch <http://bugs.python.org/issue2292>, taking into account that the comprehension part needs to be removed. -- --Guido van Rossum (python.org/~guido)

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PEP 1, PEP Purpose and Guidelines
by barry＠zope.com May 18, 2021

May 18, 2021

It has been a while since I posted a copy of PEP 1 to the mailing lists and newsgroups. I've recently done some updating of a few sections, so in the interest of gaining wider community participation in the Python development process, I'm posting the latest revision of PEP 1 here. A version of the PEP is always available on-line at http://www.python.org/peps/pep-0001.html Enjoy, -Barry -------------------- snip snip -------------------- PEP: 1 Title: PEP Purpose and Guidelines Version: $Revision: 1.36 $ Last-Modified: $Date: 2002/07/29 18:34:59 $ Author: Barry A. Warsaw, Jeremy Hylton Status: Active Type: Informational Created: 13-Jun-2000 Post-History: 21-Mar-2001, 29-Jul-2002 What is a PEP? PEP stands for Python Enhancement Proposal. A PEP is a design document providing information to the Python community, or describing a new feature for Python. The PEP should provide a concise technical specification of the feature and a rationale for the feature. We intend PEPs to be the primary mechanisms for proposing new features, for collecting community input on an issue, and for documenting the design decisions that have gone into Python. The PEP author is responsible for building consensus within the community and documenting dissenting opinions. Because the PEPs are maintained as plain text files under CVS control, their revision history is the historical record of the feature proposal[1]. Kinds of PEPs There are two kinds of PEPs. A standards track PEP describes a new feature or implementation for Python. An informational PEP describes a Python design issue, or provides general guidelines or information to the Python community, but does not propose a new feature. Informational PEPs do not necessarily represent a Python community consensus or recommendation, so users and implementors are free to ignore informational PEPs or follow their advice. PEP Work Flow The PEP editor, Barry Warsaw <peps(a)python.org>, assigns numbers for each PEP and changes its status. The PEP process begins with a new idea for Python. It is highly recommended that a single PEP contain a single key proposal or new idea. The more focussed the PEP, the more successfully it tends to be. The PEP editor reserves the right to reject PEP proposals if they appear too unfocussed or too broad. If in doubt, split your PEP into several well-focussed ones. Each PEP must have a champion -- someone who writes the PEP using the style and format described below, shepherds the discussions in the appropriate forums, and attempts to build community consensus around the idea. The PEP champion (a.k.a. Author) should first attempt to ascertain whether the idea is PEP-able. Small enhancements or patches often don't need a PEP and can be injected into the Python development work flow with a patch submission to the SourceForge patch manager[2] or feature request tracker[3]. The PEP champion then emails the PEP editor <peps(a)python.org> with a proposed title and a rough, but fleshed out, draft of the PEP. This draft must be written in PEP style as described below. If the PEP editor approves, he will assign the PEP a number, label it as standards track or informational, give it status 'draft', and create and check-in the initial draft of the PEP. The PEP editor will not unreasonably deny a PEP. Reasons for denying PEP status include duplication of effort, being technically unsound, not providing proper motivation or addressing backwards compatibility, or not in keeping with the Python philosophy. The BDFL (Benevolent Dictator for Life, Guido van Rossum) can be consulted during the approval phase, and is the final arbitrator of the draft's PEP-ability. If a pre-PEP is rejected, the author may elect to take the pre-PEP to the comp.lang.python newsgroup (a.k.a. python-list(a)python.org mailing list) to help flesh it out, gain feedback and consensus from the community at large, and improve the PEP for re-submission. The author of the PEP is then responsible for posting the PEP to the community forums, and marshaling community support for it. As updates are necessary, the PEP author can check in new versions if they have CVS commit permissions, or can email new PEP versions to the PEP editor for committing. Standards track PEPs consists of two parts, a design document and a reference implementation. The PEP should be reviewed and accepted before a reference implementation is begun, unless a reference implementation will aid people in studying the PEP. Standards Track PEPs must include an implementation - in the form of code, patch, or URL to same - before it can be considered Final. PEP authors are responsible for collecting community feedback on a PEP before submitting it for review. A PEP that has not been discussed on python-list(a)python.org and/or python-dev(a)python.org will not be accepted. However, wherever possible, long open-ended discussions on public mailing lists should be avoided. Strategies to keep the discussions efficient include, setting up a separate SIG mailing list for the topic, having the PEP author accept private comments in the early design phases, etc. PEP authors should use their discretion here. Once the authors have completed a PEP, they must inform the PEP editor that it is ready for review. PEPs are reviewed by the BDFL and his chosen consultants, who may accept or reject a PEP or send it back to the author(s) for revision. Once a PEP has been accepted, the reference implementation must be completed. When the reference implementation is complete and accepted by the BDFL, the status will be changed to `Final.' A PEP can also be assigned status `Deferred.' The PEP author or editor can assign the PEP this status when no progress is being made on the PEP. Once a PEP is deferred, the PEP editor can re-assign it to draft status. A PEP can also be `Rejected'. Perhaps after all is said and done it was not a good idea. It is still important to have a record of this fact. PEPs can also be replaced by a different PEP, rendering the original obsolete. This is intended for Informational PEPs, where version 2 of an API can replace version 1. PEP work flow is as follows: Draft -> Accepted -> Final -> Replaced ^ +----> Rejected v Deferred Some informational PEPs may also have a status of `Active' if they are never meant to be completed. E.g. PEP 1. What belongs in a successful PEP? Each PEP should have the following parts: 1. Preamble -- RFC822 style headers containing meta-data about the PEP, including the PEP number, a short descriptive title (limited to a maximum of 44 characters), the names, and optionally the contact info for each author, etc. 2. Abstract -- a short (~200 word) description of the technical issue being addressed. 3. Copyright/public domain -- Each PEP must either be explicitly labelled as placed in the public domain (see this PEP as an example) or licensed under the Open Publication License[4]. 4. Specification -- The technical specification should describe the syntax and semantics of any new language feature. The specification should be detailed enough to allow competing, interoperable implementations for any of the current Python platforms (CPython, JPython, Python .NET). 5. Motivation -- The motivation is critical for PEPs that want to change the Python language. It should clearly explain why the existing language specification is inadequate to address the problem that the PEP solves. PEP submissions without sufficient motivation may be rejected outright. 6. Rationale -- The rationale fleshes out the specification by describing what motivated the design and why particular design decisions were made. It should describe alternate designs that were considered and related work, e.g. how the feature is supported in other languages. The rationale should provide evidence of consensus within the community and discuss important objections or concerns raised during discussion. 7. Backwards Compatibility -- All PEPs that introduce backwards incompatibilities must include a section describing these incompatibilities and their severity. The PEP must explain how the author proposes to deal with these incompatibilities. PEP submissions without a sufficient backwards compatibility treatise may be rejected outright. 8. Reference Implementation -- The reference implementation must be completed before any PEP is given status 'Final,' but it need not be completed before the PEP is accepted. It is better to finish the specification and rationale first and reach consensus on it before writing code. The final implementation must include test code and documentation appropriate for either the Python language reference or the standard library reference. PEP Template PEPs are written in plain ASCII text, and should adhere to a rigid style. There is a Python script that parses this style and converts the plain text PEP to HTML for viewing on the web[5]. PEP 9 contains a boilerplate[7] template you can use to get started writing your PEP. Each PEP must begin with an RFC822 style header preamble. The headers must appear in the following order. Headers marked with `*' are optional and are described below. All other headers are required. PEP: <pep number> Title: <pep title> Version: <cvs version string> Last-Modified: <cvs date string> Author: <list of authors' real names and optionally, email addrs> * Discussions-To: <email address> Status: <Draft | Active | Accepted | Deferred | Final | Replaced> Type: <Informational | Standards Track> * Requires: <pep numbers> Created: <date created on, in dd-mmm-yyyy format> * Python-Version: <version number> Post-History: <dates of postings to python-list and python-dev> * Replaces: <pep number> * Replaced-By: <pep number> The Author: header lists the names and optionally, the email addresses of all the authors/owners of the PEP. The format of the author entry should be address(a)dom.ain (Random J. User) if the email address is included, and just Random J. User if the address is not given. If there are multiple authors, each should be on a separate line following RFC 822 continuation line conventions. Note that personal email addresses in PEPs will be obscured as a defense against spam harvesters. Standards track PEPs must have a Python-Version: header which indicates the version of Python that the feature will be released with. Informational PEPs do not need a Python-Version: header. While a PEP is in private discussions (usually during the initial Draft phase), a Discussions-To: header will indicate the mailing list or URL where the PEP is being discussed. No Discussions-To: header is necessary if the PEP is being discussed privately with the author, or on the python-list or python-dev email mailing lists. Note that email addresses in the Discussions-To: header will not be obscured. Created: records the date that the PEP was assigned a number, while Post-History: is used to record the dates of when new versions of the PEP are posted to python-list and/or python-dev. Both headers should be in dd-mmm-yyyy format, e.g. 14-Aug-2001. PEPs may have a Requires: header, indicating the PEP numbers that this PEP depends on. PEPs may also have a Replaced-By: header indicating that a PEP has been rendered obsolete by a later document; the value is the number of the PEP that replaces the current document. The newer PEP must have a Replaces: header containing the number of the PEP that it rendered obsolete. PEP Formatting Requirements PEP headings must begin in column zero and the initial letter of each word must be capitalized as in book titles. Acronyms should be in all capitals. The body of each section must be indented 4 spaces. Code samples inside body sections should be indented a further 4 spaces, and other indentation can be used as required to make the text readable. You must use two blank lines between the last line of a section's body and the next section heading. You must adhere to the Emacs convention of adding two spaces at the end of every sentence. You should fill your paragraphs to column 70, but under no circumstances should your lines extend past column 79. If your code samples spill over column 79, you should rewrite them. Tab characters must never appear in the document at all. A PEP should include the standard Emacs stanza included by example at the bottom of this PEP. A PEP must contain a Copyright section, and it is strongly recommended to put the PEP in the public domain. When referencing an external web page in the body of a PEP, you should include the title of the page in the text, with a footnote reference to the URL. Do not include the URL in the body text of the PEP. E.g. Refer to the Python Language web site [1] for more details. ... [1] http://www.python.org When referring to another PEP, include the PEP number in the body text, such as "PEP 1". The title may optionally appear. Add a footnote reference that includes the PEP's title and author. It may optionally include the explicit URL on a separate line, but only in the References section. Note that the pep2html.py script will calculate URLs automatically, e.g.: ... Refer to PEP 1 [7] for more information about PEP style ... References [7] PEP 1, PEP Purpose and Guidelines, Warsaw, Hylton http://www.python.org/peps/pep-0001.html If you decide to provide an explicit URL for a PEP, please use this as the URL template: http://www.python.org/peps/pep-xxxx.html PEP numbers in URLs must be padded with zeros from the left, so as to be exactly 4 characters wide, however PEP numbers in text are never padded. Reporting PEP Bugs, or Submitting PEP Updates How you report a bug, or submit a PEP update depends on several factors, such as the maturity of the PEP, the preferences of the PEP author, and the nature of your comments. For the early draft stages of the PEP, it's probably best to send your comments and changes directly to the PEP author. For more mature, or finished PEPs you may want to submit corrections to the SourceForge bug manager[6] or better yet, the SourceForge patch manager[2] so that your changes don't get lost. If the PEP author is a SF developer, assign the bug/patch to him, otherwise assign it to the PEP editor. When in doubt about where to send your changes, please check first with the PEP author and/or PEP editor. PEP authors who are also SF committers, can update the PEPs themselves by using "cvs commit" to commit their changes. Remember to also push the formatted PEP text out to the web by doing the following: % python pep2html.py -i NUM where NUM is the number of the PEP you want to push out. See % python pep2html.py --help for details. Transferring PEP Ownership It occasionally becomes necessary to transfer ownership of PEPs to a new champion. In general, we'd like to retain the original author as a co-author of the transferred PEP, but that's really up to the original author. A good reason to transfer ownership is because the original author no longer has the time or interest in updating it or following through with the PEP process, or has fallen off the face of the 'net (i.e. is unreachable or not responding to email). A bad reason to transfer ownership is because you don't agree with the direction of the PEP. We try to build consensus around a PEP, but if that's not possible, you can always submit a competing PEP. If you are interested assuming ownership of a PEP, send a message asking to take over, addressed to both the original author and the PEP editor <peps(a)python.org>. If the original author doesn't respond to email in a timely manner, the PEP editor will make a unilateral decision (it's not like such decisions can be reversed. :). References and Footnotes [1] This historical record is available by the normal CVS commands for retrieving older revisions. For those without direct access to the CVS tree, you can browse the current and past PEP revisions via the SourceForge web site at http://cvs.sourceforge.net/cgi-bin/cvsweb.cgi/python/nondist/peps/?cvsroot=… [2] http://sourceforge.net/tracker/?group_id=5470&atid=305470 [3] http://sourceforge.net/tracker/?atid=355470&group_id=5470&func=browse [4] http://www.opencontent.org/openpub/ [5] The script referred to here is pep2html.py, which lives in the same directory in the CVS tree as the PEPs themselves. Try "pep2html.py --help" for details. The URL for viewing PEPs on the web is http://www.python.org/peps/ [6] http://sourceforge.net/tracker/?group_id=5470&atid=305470 [7] PEP 9, Sample PEP Template http://www.python.org/peps/pep-0009.html Copyright This document has been placed in the public domain. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 End:

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Re: [Python-Dev] Status on PEP-431 Timezones
by Isaac Schwabacher July 30, 2015

July 30, 2015

On 15-04-15, Akira Li <4kir4.1i(a)gmail.com> wrote: > Isaac Schwabacher <ischwabacher(a)wisc.edu> writes: > > > On 15-04-15, Akira Li <4kir4.1i(a)gmail.com> wrote: > >> Isaac Schwabacher <ischwabacher(a)wisc.edu> writes: > >> > ... > >> > > >> > I know that you can do datetime.now(tz), and you can do datetime(2013, > >> > 11, 3, 1, 30, tzinfo=zoneinfo('America/Chicago')), but not being able > >> > to add a time zone to an existing naive datetime is painful (and > >> > strptime doesn't even let you pass in a time zone). > >> > >> `.now(tz)` is correct. `datetime(..., tzinfo=tz)`) is wrong: if tz is a > >> pytz timezone then you may get a wrong tzinfo (LMT), you should use > >> `tz.localize(naive_dt, is_dst=False|True|None)` instead. > > > > The whole point of this thread is to finalize PEP 431, which fixes the > > problem for which `localize()` and `normalize()` are workarounds. When > > this is done, `datetime(..., tzinfo=tz)` will be correct. > > > > ijs > > The input time is ambiguous. Even if we assume PEP 431 is implemented in > some form, your code is still missing isdst parameter (or the > analog). PEP 431 won't fix it; it can't resolve the ambiguity by > itself. Notice is_dst paramter in the `tz.localize()` call (current > API). ...yeah, I forgot to throw that in there. It was supposed to be there all along. Nothing to see here, move along. > .now(tz) works even during end-of-DST transitions (current API) when the > local time is ambiguous. I know that. That's what I was complaining about-- I was trying to talk about how astimezone() was going to be inadequate even after the PEP was implemented because it couldn't turn naive datetimes into aware ones, and people were giving examples that started with aware datetimes generated by now(tz), which completely went around the point I was trying to make. But it looks like astimezone() is going to grow an is_dst parameter, and everything will be OK. ijs

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Clarification of PEP 476 "opting out" section
by Nick Coghlan May 12, 2015

May 12, 2015

Hi folks, This is just a note to highlight the fact that I tweaked the "Opting out" section in PEP 476 based on various discussions I've had over the past few months: https://hg.python.org/peps/rev/dfd96ee9d6a8 The notable changes: * the example monkeypatching code handles AttributeError when looking up "ssl._create_unverified_context", in order to accommodate older versions of Python that don't have PEP 476 implemented * new paragraph making it clearer that while the intended use case for the monkeypatching trick is as a workaround to handle environments where you *know* HTTPS certificate verification won't work properly (including explicit references to sitecustomize.py and Standard Operating Environments for Python), there's also a secondary use case in allowing applications to provide a system administrator controlled setting to globally disable certificate verification (hence the change to the example code) * new paragraph making it explicit that even though we've improved Python's default behaviour, particularly security sensitive applications should still provide their own context rather than relying on the defaults Regards, Nick. -- Nick Coghlan | ncoghlan(a)gmail.com | Brisbane, Australia

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What's missing in PEP-484 (Type hints)
by Dima Tisnek May 12, 2015

May 12, 2015

# Syntactic sugar "Beautiful is better than ugly, " thus nice syntax is needed. Current syntax is very mechanical. Syntactic sugar is needed on top of current PEP. # internal vs external @intify def foo() -> int: b = "42" return b # check 1 x = foo() // 2 # check 2 Does the return type apply to implementation (str) or decorated callable (int)? How can same annotation or a pair of annotations be used to: * validate return statement type * validate subsequent use * look reasonable in the source code # lambda Not mentioned in the PEP, omitted for convenience or is there a rationale? f = lambda x: None if x is None else str(x ** 2) Current syntax seems to preclude annotation of `x` due to colon. Current syntax sort of allows lamba return type annotation, but it's easy to confuse with `f`. # local variables Not mentioned in the PEP Non-trivial code could really use these. # global variables Not mentioned in the PEP Module-level globals are part of API, annotation is welcome. What is the syntax? # comprehensions [3 * x.data for x in foo if "bar" in x.type] Arguable, perhaps annotation is only needed on `foo` here, but then how complex comprehensions, e.g. below, the intermediate comprehension could use an annotation [xx for y in [...] if ...] # class attributes s = socket.socket(...) s.type, s.family, s.proto # int s.fileno # callable If annotations are only available for methods, it will lead to Java-style explicit getters and setters. Python language and data model prefers properties instead, thus annotations are needed on attributes. # plain data user1 = dict(id=123, # always int name="uuu", # always str ...) # other fields possible smth = [42, "xx", ...] (why not namedtuple? b/c extensible, mutable) At least one PHP IDE allows to annotate PDO. Perhaps it's just bad taste in Python? Or is there a valid use-case? # personal note I think it's amazing how much thought has already been put into this proposal. The foundation is pretty solid (per Guido talk). I am not at all opposed to software that infers types (like jedi), or reads user-specified types (like phpstorm and pep 484) and does something good with that. In fact I'm ambivalent to current proposal, standard and promise of better tools on one hand; narrow scope, debatable looks on the other. -- dima

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Unicode literals in Python 2.7
by Adam Bartoš May 12, 2015

May 12, 2015

Hello, is it possible to somehow tell Python 2.7 to compile a code entered in the interactive session with the flag PyCF_SOURCE_IS_UTF8 set? I'm considering adding support for Python 2 in my package ( https://github.com/Drekin/win-unicode-console) and I have run into the fact that when u"α" is entered in the interactive session, it results in u"\xce\xb1" rather than u"\u03b1". As this seems to be a highly specialized question, I'm asking it here. Regards, Drekin

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PEP 492 quibble and request
by Ethan Furman May 7, 2015

May 7, 2015

>From the PEP: > Why not a __future__ import > > __future__ imports are inconvenient and easy to forget to add. That is a horrible rationale for not using an import. By that logic we should have everything in built-ins. ;) > Working example > ... The working example only uses async def and await, not async with nor async for nor __aenter__, etc., etc. Could you put in a more complete example -- maybe a basic chat room with both server and client -- that demonstrated more of the new possibilities? Having gone through the PEP again, I am still no closer to understanding what happens here: data = await reader.read(8192) What does the flow of control look like at the interpreter level? -- ~Ethan~

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PEP 492: async/await in Python; version 4
by Yury Selivanov May 6, 2015

May 6, 2015

Hi python-dev, New version of the PEP is attached. Summary of updates: 1. Terminology: - *native coroutine* term is used for "async def" functions. - *generator-based coroutine* term is used for PEP 380 coroutines used in asyncio. - *coroutine* is used when *native coroutine* or *generator based coroutine* can be used in the same context. I think that it's not really productive to discuss the terminology that we use in the PEP. Its only purpose is to disambiguate concepts used in the PEP. We should discuss how we will name new 'async def' coroutines in Python Documentation if the PEP is accepted. Although if you notice that somewhere in the PEP it is not crystal clear what "coroutine" means please give me a heads up! 2. Syntax of await expressions is now thoroghly defined in the PEP. See "Await Expression", "Updated operator precedence table", and "Examples of "await" expressions" sections. I like the current approach. I'm still not convinced that we should make 'await' the same grammatically as unary minus. I don't understand why we should allow parsing things like 'await -fut'; this should be a SyntaxError. I'm fine to modify the grammar to allow 'await await fut' syntax, though. And I'm open to discussion :) 3. CO_NATIVE_COROUTINE flag. This enables us to disable __iter__ and __next__ on native coroutines while maintaining full backwards compatibility. 4. asyncio / Migration strategy. Existing code can be used with PEP 492 as is, everything will work as expected. However, since *plain generator*s (not decorated with @asyncio.coroutine) cannot 'yield from' native coroutines (async def functions), it might break some code *while adapting it to the new syntax*. I'm open to just throw a RuntimeWarning in this case in 3.5, and raise a TypeError in 3.6. Please see the "Differences from generators" section of the PEP. 5. inspect.isawaitable() function. And, all new functions are now listed in the "New Standard Library Functions" section. 6. Lot's of small updates and tweaks throughout the PEP. Thanks, Yury PEP: 492 Title: Coroutines with async and await syntax Version: $Revision$ Last-Modified: $Date$ Author: Yury Selivanov <yselivanov(a)sprymix.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 09-Apr-2015 Python-Version: 3.5 Post-History: 17-Apr-2015, 21-Apr-2015, 27-Apr-2015, 29-Apr-2015 Abstract ======== This PEP introduces new syntax for coroutines, asynchronous ``with`` statements and ``for`` loops. The main motivation behind this proposal is to streamline writing and maintaining asynchronous code, as well as to simplify previously hard to implement code patterns. Rationale and Goals =================== Current Python supports implementing coroutines via generators (PEP 342), further enhanced by the ``yield from`` syntax introduced in PEP 380. This approach has a number of shortcomings: * it is easy to confuse coroutines with regular generators, since they share the same syntax; async libraries often attempt to alleviate this by using decorators (e.g. ``(a)asyncio.coroutine`` [1]_); * it is not possible to natively define a coroutine which has no ``yield`` or ``yield from`` statements, again requiring the use of decorators to fix potential refactoring issues; * support for asynchronous calls is limited to expressions where ``yield`` is allowed syntactically, limiting the usefulness of syntactic features, such as ``with`` and ``for`` statements. This proposal makes coroutines a native Python language feature, and clearly separates them from generators. This removes generator/coroutine ambiguity, and makes it possible to reliably define coroutines without reliance on a specific library. This also enables linters and IDEs to improve static code analysis and refactoring. Native coroutines and the associated new syntax features make it possible to define context manager and iteration protocols in asynchronous terms. As shown later in this proposal, the new ``async with`` statement lets Python programs perform asynchronous calls when entering and exiting a runtime context, and the new ``async for`` statement makes it possible to perform asynchronous calls in iterators. Specification ============= This proposal introduces new syntax and semantics to enhance coroutine support in Python. This specification presumes knowledge of the implementation of coroutines in Python (PEP 342 and PEP 380). Motivation for the syntax changes proposed here comes from the asyncio framework (PEP 3156) and the "Cofunctions" proposal (PEP 3152, now rejected in favor of this specification). From this point in this document we use the word *native coroutine* to refer to functions declared using the new syntax. *generator-based coroutine* is used where necessary to refer to coroutines that are based on generator syntax. *coroutine* is used in contexts where both definitions are applicable. New Coroutine Declaration Syntax -------------------------------- The following new syntax is used to declare a *native coroutine*:: async def read_data(db): pass Key properties of *native coroutines*: * ``async def`` functions are always coroutines, even if they do not contain ``await`` expressions. * It is a ``SyntaxError`` to have ``yield`` or ``yield from`` expressions in an ``async`` function. * Internally, two new code object flags were introduced: - ``CO_COROUTINE`` is used to enable runtime detection of *coroutines* (and migrating existing code). - ``CO_NATIVE_COROUTINE`` is used to mark *native coroutines* (defined with new syntax.) All coroutines have ``CO_COROUTINE``, ``CO_NATIVE_COROUTINE``, and ``CO_GENERATOR`` flags set. * Regular generators, when called, return a *generator object*; similarly, coroutines return a *coroutine object*. * ``StopIteration`` exceptions are not propagated out of coroutines, and are replaced with a ``RuntimeError``. For regular generators such behavior requires a future import (see PEP 479). * See also `Coroutine objects`_ section. types.coroutine() ----------------- A new function ``coroutine(gen)`` is added to the ``types`` module. It allows interoperability between existing *generator-based coroutines* in asyncio and *native coroutines* introduced by this PEP. The function applies ``CO_COROUTINE`` flag to generator-function's code object, making it return a *coroutine object*. The function can be used as a decorator, since it modifies generator- functions in-place and returns them. Note, that the ``CO_NATIVE_COROUTINE`` flag is not applied by ``types.coroutine()`` to make it possible to separate *native coroutines* defined with new syntax, from *generator-based coroutines*. Await Expression ---------------- The following new ``await`` expression is used to obtain a result of coroutine execution:: async def read_data(db): data = await db.fetch('SELECT ...') ... ``await``, similarly to ``yield from``, suspends execution of ``read_data`` coroutine until ``db.fetch`` *awaitable* completes and returns the result data. It uses the ``yield from`` implementation with an extra step of validating its argument. ``await`` only accepts an *awaitable*, which can be one of: * A *native coroutine object* returned from a *native coroutine*. * A *generator-based coroutine object* returned from a generator decorated with ``types.coroutine()``. * An object with an ``__await__`` method returning an iterator. Any ``yield from`` chain of calls ends with a ``yield``. This is a fundamental mechanism of how *Futures* are implemented. Since, internally, coroutines are a special kind of generators, every ``await`` is suspended by a ``yield`` somewhere down the chain of ``await`` calls (please refer to PEP 3156 for a detailed explanation.) To enable this behavior for coroutines, a new magic method called ``__await__`` is added. In asyncio, for instance, to enable *Future* objects in ``await`` statements, the only change is to add ``__await__ = __iter__`` line to ``asyncio.Future`` class. Objects with ``__await__`` method are called *Future-like* objects in the rest of this PEP. Also, please note that ``__aiter__`` method (see its definition below) cannot be used for this purpose. It is a different protocol, and would be like using ``__iter__`` instead of ``__call__`` for regular callables. It is a ``TypeError`` if ``__await__`` returns anything but an iterator. * Objects defined with CPython C API with a ``tp_await`` function, returning an iterator (similar to ``__await__`` method). It is a ``SyntaxError`` to use ``await`` outside of an ``async def`` function (like it is a ``SyntaxError`` to use ``yield`` outside of ``def`` function.) It is a ``TypeError`` to pass anything other than an *awaitable* object to an ``await`` expression. Updated operator precedence table ''''''''''''''''''''''''''''''''' ``await`` keyword is defined as follows:: power ::= await ["**" u_expr] await ::= ["await"] primary where "primary" represents the most tightly bound operations of the language. Its syntax is:: primary ::= atom | attributeref | subscription | slicing | call See Python Documentation [12]_ and `Grammar Updates`_ section of this proposal for details. The key ``await`` difference from ``yield`` and ``yield from`` operators is that *await expressions* do not require parentheses around them most of the times. Also, ``yield from`` allows any expression as its argument, including expressions like ``yield from a() + b()``, that would be parsed as ``yield from (a() + b())``, which is almost always a bug. In general, the result of any arithmetic operation is not an *awaitable* object. To avoid this kind of mistakes, it was decided to make ``await`` precedence lower than ``[]``, ``()``, and ``.``, but higher than ``**`` operators. +------------------------------+-----------------------------------+ | Operator | Description | +==============================+===================================+ | ``yield`` ``x``, | Yield expression | | ``yield from`` ``x`` | | +------------------------------+-----------------------------------+ | ``lambda`` | Lambda expression | +------------------------------+-----------------------------------+ | ``if`` -- ``else`` | Conditional expression | +------------------------------+-----------------------------------+ | ``or`` | Boolean OR | +------------------------------+-----------------------------------+ | ``and`` | Boolean AND | +------------------------------+-----------------------------------+ | ``not`` ``x`` | Boolean NOT | +------------------------------+-----------------------------------+ | ``in``, ``not in``, | Comparisons, including membership | | ``is``, ``is not``, ``<``, | tests and identity tests | | ``<=``, ``>``, ``>=``, | | | ``!=``, ``==`` | | +------------------------------+-----------------------------------+ | ``|`` | Bitwise OR | +------------------------------+-----------------------------------+ | ``^`` | Bitwise XOR | +------------------------------+-----------------------------------+ | ``&`` | Bitwise AND | +------------------------------+-----------------------------------+ | ``<<``, ``>>`` | Shifts | +------------------------------+-----------------------------------+ | ``+``, ``-`` | Addition and subtraction | +------------------------------+-----------------------------------+ | ``*``, ``@``, ``/``, ``//``, | Multiplication, matrix | | ``%`` | multiplication, division, | | | remainder | +------------------------------+-----------------------------------+ | ``+x``, ``-x``, ``~x`` | Positive, negative, bitwise NOT | +------------------------------+-----------------------------------+ | ``**`` | Exponentiation | +------------------------------+-----------------------------------+ | ``await`` ``x`` | Await expression | +------------------------------+-----------------------------------+ | ``x[index]``, | Subscription, slicing, | | ``x[index:index]``, | call, attribute reference | | ``x(arguments...)``, | | | ``x.attribute`` | | +------------------------------+-----------------------------------+ | ``(expressions...)``, | Binding or tuple display, | | ``[expressions...]``, | list display, | | ``{key: value...}``, | dictionary display, | | ``{expressions...}`` | set display | +------------------------------+-----------------------------------+ Examples of "await" expressions ''''''''''''''''''''''''''''''' Valid syntax examples: ================================== ================================== Expression Will be parsed as ================================== ================================== ``if await fut: pass`` ``if (await fut): pass`` ``if await fut + 1: pass`` ``if (await fut) + 1: pass`` ``pair = await fut, 'spam'`` ``pair = (await fut), 'spam'`` ``with await fut, open(): pass`` ``with (await fut), open(): pass`` ``await foo()['spam'].baz()()`` ``await ( foo()['spam'].baz()() )`` ``return await coro()`` ``return ( await coro() )`` ``res = await coro() ** 2`` ``res = (await coro()) ** 2`` ``func(a1=await coro(), a2=0)`` ``func(a1=(await coro()), a2=0)`` ``await foo() + await bar()`` ``(await foo()) + (await bar())`` ``-await foo()`` ``-(await foo())`` ================================== ================================== Invalid syntax examples: ================================== ================================== Expression Should be written as ================================== ================================== ``await await coro()`` ``await (await coro())`` ``await -coro()`` ``await (-coro())`` ================================== ================================== Asynchronous Context Managers and "async with" ---------------------------------------------- An *asynchronous context manager* is a context manager that is able to suspend execution in its *enter* and *exit* methods. To make this possible, a new protocol for asynchronous context managers is proposed. Two new magic methods are added: ``__aenter__`` and ``__aexit__``. Both must return an *awaitable*. An example of an asynchronous context manager:: class AsyncContextManager: async def __aenter__(self): await log('entering context') async def __aexit__(self, exc_type, exc, tb): await log('exiting context') New Syntax '''''''''' A new statement for asynchronous context managers is proposed:: async with EXPR as VAR: BLOCK which is semantically equivalent to:: mgr = (EXPR) aexit = type(mgr).__aexit__ aenter = type(mgr).__aenter__(mgr) exc = True try: VAR = await aenter BLOCK except: if not await aexit(mgr, *sys.exc_info()): raise else: await aexit(mgr, None, None, None) As with regular ``with`` statements, it is possible to specify multiple context managers in a single ``async with`` statement. It is an error to pass a regular context manager without ``__aenter__`` and ``__aexit__`` methods to ``async with``. It is a ``SyntaxError`` to use ``async with`` outside of an ``async def`` function. Example ''''''' With *asynchronous context managers* it is easy to implement proper database transaction managers for coroutines:: async def commit(session, data): ... async with session.transaction(): ... await session.update(data) ... Code that needs locking also looks lighter:: async with lock: ... instead of:: with (yield from lock): ... Asynchronous Iterators and "async for" -------------------------------------- An *asynchronous iterable* is able to call asynchronous code in its *iter* implementation, and *asynchronous iterator* can call asynchronous code in its *next* method. To support asynchronous iteration: 1. An object must implement an ``__aiter__`` method returning an *awaitable* resulting in an *asynchronous iterator object*. 2. An *asynchronous iterator object* must implement an ``__anext__`` method returning an *awaitable*. 3. To stop iteration ``__anext__`` must raise a ``StopAsyncIteration`` exception. An example of asynchronous iterable:: class AsyncIterable: async def __aiter__(self): return self async def __anext__(self): data = await self.fetch_data() if data: return data else: raise StopAsyncIteration async def fetch_data(self): ... New Syntax '''''''''' A new statement for iterating through asynchronous iterators is proposed:: async for TARGET in ITER: BLOCK else: BLOCK2 which is semantically equivalent to:: iter = (ITER) iter = await type(iter).__aiter__(iter) running = True while running: try: TARGET = await type(iter).__anext__(iter) except StopAsyncIteration: running = False else: BLOCK else: BLOCK2 It is a ``TypeError`` to pass a regular iterable without ``__aiter__`` method to ``async for``. It is a ``SyntaxError`` to use ``async for`` outside of an ``async def`` function. As for with regular ``for`` statement, ``async for`` has an optional ``else`` clause. Example 1 ''''''''' With asynchronous iteration protocol it is possible to asynchronously buffer data during iteration:: async for data in cursor: ... Where ``cursor`` is an asynchronous iterator that prefetches ``N`` rows of data from a database after every ``N`` iterations. The following code illustrates new asynchronous iteration protocol:: class Cursor: def __init__(self): self.buffer = collections.deque() def _prefetch(self): ... async def __aiter__(self): return self async def __anext__(self): if not self.buffer: self.buffer = await self._prefetch() if not self.buffer: raise StopAsyncIteration return self.buffer.popleft() then the ``Cursor`` class can be used as follows:: async for row in Cursor(): print(row) which would be equivalent to the following code:: i = await Cursor().__aiter__() while True: try: row = await i.__anext__() except StopAsyncIteration: break else: print(row) Example 2 ''''''''' The following is a utility class that transforms a regular iterable to an asynchronous one. While this is not a very useful thing to do, the code illustrates the relationship between regular and asynchronous iterators. :: class AsyncIteratorWrapper: def __init__(self, obj): self._it = iter(obj) async def __aiter__(self): return self async def __anext__(self): try: value = next(self._it) except StopIteration: raise StopAsyncIteration return value async for letter in AsyncIteratorWrapper("abc"): print(letter) Why StopAsyncIteration? ''''''''''''''''''''''' Coroutines are still based on generators internally. So, before PEP 479, there was no fundamental difference between :: def g1(): yield from fut return 'spam' and :: def g2(): yield from fut raise StopIteration('spam') And since PEP 479 is accepted and enabled by default for coroutines, the following example will have its ``StopIteration`` wrapped into a ``RuntimeError`` :: async def a1(): await fut raise StopIteration('spam') The only way to tell the outside code that the iteration has ended is to raise something other than ``StopIteration``. Therefore, a new built-in exception class ``StopAsyncIteration`` was added. Moreover, with semantics from PEP 479, all ``StopIteration`` exceptions raised in coroutines are wrapped in ``RuntimeError``. Coroutine objects ----------------- Differences from generators ''''''''''''''''''''''''''' This section applies only to *native coroutines* with ``CO_NATIVE_COROUTINE`` flag, i.e. defined with the new ``async def`` syntax. **The behavior of existing *generator-based coroutines* in asyncio remains unchanged.** Great effort has been made to make sure that coroutines and generators are treated as distinct concepts: 1. *Native coroutine objects* do not implement ``__iter__`` and ``__next__`` methods. Therefore, they cannot be iterated over or passed to ``iter()``, ``list()``, ``tuple()`` and other built-ins. They also cannot be used in a ``for..in`` loop. An attempt to use ``__iter__`` or ``__next__`` on a *native coroutine object* will result in a ``TypeError``. 2. *Plain generators* cannot ``yield from`` *native coroutine objects*: doing so will result in a ``TypeError``. 3. *generator-based coroutines* (for asyncio code must be decorated with ``(a)asyncio.coroutine``) can ``yield from`` *native coroutine objects*. 4. ``inspect.isgenerator()`` and ``inspect.isgeneratorfunction()`` return ``False`` for *native coroutine objects* and *native coroutine functions*. Coroutine object methods '''''''''''''''''''''''' Coroutines are based on generators internally, thus they share the implementation. Similarly to generator objects, coroutine objects have ``throw()``, ``send()`` and ``close()`` methods. ``StopIteration`` and ``GeneratorExit`` play the same role for coroutine objects (although PEP 479 is enabled by default for coroutines). See PEP 342, PEP 380, and Python Documentation [11]_ for details. ``throw()``, ``send()`` methods for coroutine objects are used to push values and raise errors into *Future-like* objects. Debugging Features ------------------ A common beginner mistake is forgetting to use ``yield from`` on coroutines:: @asyncio.coroutine def useful(): asyncio.sleep(1) # this will do noting without 'yield from' For debugging this kind of mistakes there is a special debug mode in asyncio, in which ``@coroutine`` decorator wraps all functions with a special object with a destructor logging a warning. Whenever a wrapped generator gets garbage collected, a detailed logging message is generated with information about where exactly the decorator function was defined, stack trace of where it was collected, etc. Wrapper object also provides a convenient ``__repr__`` function with detailed information about the generator. The only problem is how to enable these debug capabilities. Since debug facilities should be a no-op in production mode, ``@coroutine`` decorator makes the decision of whether to wrap or not to wrap based on an OS environment variable ``PYTHONASYNCIODEBUG``. This way it is possible to run asyncio programs with asyncio's own functions instrumented. ``EventLoop.set_debug``, a different debug facility, has no impact on ``@coroutine`` decorator's behavior. With this proposal, coroutines is a native, distinct from generators, concept. New methods ``set_coroutine_wrapper`` and ``get_coroutine_wrapper`` are added to the ``sys`` module, with which frameworks can provide advanced debugging facilities. It is also important to make coroutines as fast and efficient as possible, therefore there are no debug features enabled by default. Example:: async def debug_me(): await asyncio.sleep(1) def async_debug_wrap(generator): return asyncio.CoroWrapper(generator) sys.set_coroutine_wrapper(async_debug_wrap) debug_me() # <- this line will likely GC the coroutine object and # trigger asyncio.CoroWrapper's code. assert isinstance(debug_me(), asyncio.CoroWrapper) sys.set_coroutine_wrapper(None) # <- this unsets any # previously set wrapper assert not isinstance(debug_me(), asyncio.CoroWrapper) New Standard Library Functions ------------------------------ * ``types.coroutine(gen)``. See `types.coroutine()`_ section for details. * ``inspect.iscoroutine(obj)`` returns ``True`` if ``obj`` is a *coroutine object*. * ``inspect.iscoroutinefunction(obj)`` returns ``True`` if ``obj`` is a *coroutine function*. * ``inspect.isawaitable(obj)`` returns ``True`` if ``obj`` can be used in ``await`` expression. See `Await Expression`_ for details. * ``sys.set_coroutine_wrapper(wraper)`` allows to intercept creation of *coroutine objects*. ``wraper`` must be a callable that accepts one argument: a *coroutine object* or ``None``. ``None`` resets the wrapper. If called twice, the new wrapper replaces the previous one. See `Debugging Features`_ for more details. * ``sys.get_coroutine_wrapper()`` returns the current wrapper object. Returns ``None`` if no wrapper was set. See `Debugging Features`_ for more details. Glossary ======== :Native coroutine: A coroutine function is declared with ``async def``. It uses ``await`` and ``return value``; see `New Coroutine Declaration Syntax`_ for details. :Native coroutine object: Returned from a native coroutine function. See `Await Expression`_ for details. :Generator-based coroutine: Coroutines based on generator syntax. Most common example are functions decorated with ``(a)asyncio.coroutine``. :Generator-based coroutine object: Returned from a generator-based coroutine function. :Coroutine: Either *native coroutine* or *generator-based coroutine*. :Coroutine object: Either *native coroutine object* or *generator-based coroutine object*. :Future-like object: An object with an ``__await__`` method, or a C object with ``tp_await`` function, returning an iterator. Can be consumed by an ``await`` expression in a coroutine. A coroutine waiting for a Future-like object is suspended until the Future-like object's ``__await__`` completes, and returns the result. See `Await Expression`_ for details. :Awaitable: A *Future-like* object or a *coroutine object*. See `Await Expression`_ for details. :Asynchronous context manager: An asynchronous context manager has ``__aenter__`` and ``__aexit__`` methods and can be used with ``async with``. See `Asynchronous Context Managers and "async with"`_ for details. :Asynchronous iterable: An object with an ``__aiter__`` method, which must return an *asynchronous iterator* object. Can be used with ``async for``. See `Asynchronous Iterators and "async for"`_ for details. :Asynchronous iterator: An asynchronous iterator has an ``__anext__`` method. See `Asynchronous Iterators and "async for"`_ for details. List of functions and methods ============================= ================= =================================== ================= Method Can contain Can't contain ================= =================================== ================= async def func await, return value yield, yield from async def __a*__ await, return value yield, yield from def __a*__ return awaitable await def __await__ yield, yield from, return iterable await generator yield, yield from, return value await ================= =================================== ================= Where: * "async def func": native coroutine; * "async def __a*__": ``__aiter__``, ``__anext__``, ``__aenter__``, ``__aexit__`` defined with the ``async`` keyword; * "def __a*__": ``__aiter__``, ``__anext__``, ``__aenter__``, ``__aexit__`` defined without the ``async`` keyword, must return an *awaitable*; * "def __await__": ``__await__`` method to implement *Future-like* objects; * generator: a "regular" generator, function defined with ``def`` and which contains a least one ``yield`` or ``yield from`` expression. Transition Plan =============== To avoid backwards compatibility issues with ``async`` and ``await`` keywords, it was decided to modify ``tokenizer.c`` in such a way, that it: * recognizes ``async def`` name tokens combination (start of a native coroutine); * keeps track of regular functions and native coroutines; * replaces ``'async'`` token with ``ASYNC`` and ``'await'`` token with ``AWAIT`` when in the process of yielding tokens for native coroutines. This approach allows for seamless combination of new syntax features (all of them available only in ``async`` functions) with any existing code. An example of having "async def" and "async" attribute in one piece of code:: class Spam: async = 42 async def ham(): print(getattr(Spam, 'async')) # The coroutine can be executed and will print '42' Backwards Compatibility ----------------------- This proposal preserves 100% backwards compatibility. asyncio ------- ``asyncio`` module was adapted and tested to work with coroutines and new statements. Backwards compatibility is 100% preserved, i.e. all existing code will work as-is. The required changes are mainly: 1. Modify ``(a)asyncio.coroutine`` decorator to use new ``types.coroutine()`` function. 2. Add ``__await__ = __iter__`` line to ``asyncio.Future`` class. 3. Add ``ensure_task()`` as an alias for ``async()`` function. Deprecate ``async()`` function. Migration strategy '''''''''''''''''' Because *plain generators* cannot ``yield from`` *native coroutine objects* (see `Differences from generators`_ section for more details), it is advised to make sure that all generator-based coroutines are decorated with ``(a)asyncio.coroutine`` *before* starting to use the new syntax. Grammar Updates --------------- Grammar changes are also fairly minimal:: decorated: decorators (classdef | funcdef | async_funcdef) async_funcdef: ASYNC funcdef compound_stmt: (if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt) async_stmt: ASYNC (funcdef | with_stmt | for_stmt) power: atom_expr ['**' factor] atom_expr: [AWAIT] atom trailer* Transition Period Shortcomings ------------------------------ There is just one. Until ``async`` and ``await`` are not proper keywords, it is not possible (or at least very hard) to fix ``tokenizer.c`` to recognize them on the **same line** with ``def`` keyword:: # async and await will always be parsed as variables async def outer(): # 1 def nested(a=(await fut)): pass async def foo(): return (await fut) # 2 Since ``await`` and ``async`` in such cases are parsed as ``NAME`` tokens, a ``SyntaxError`` will be raised. To workaround these issues, the above examples can be easily rewritten to a more readable form:: async def outer(): # 1 a_default = await fut def nested(a=a_default): pass async def foo(): # 2 return (await fut) This limitation will go away as soon as ``async`` and ``await`` are proper keywords. Or if it's decided to use a future import for this PEP. Deprecation Plans ----------------- ``async`` and ``await`` names will be softly deprecated in CPython 3.5 and 3.6. In 3.7 we will transform them to proper keywords. Making ``async`` and ``await`` proper keywords before 3.7 might make it harder for people to port their code to Python 3. Design Considerations ===================== PEP 3152 -------- PEP 3152 by Gregory Ewing proposes a different mechanism for coroutines (called "cofunctions"). Some key points: 1. A new keyword ``codef`` to declare a *cofunction*. *Cofunction* is always a generator, even if there is no ``cocall`` expressions inside it. Maps to ``async def`` in this proposal. 2. A new keyword ``cocall`` to call a *cofunction*. Can only be used inside a *cofunction*. Maps to ``await`` in this proposal (with some differences, see below.) 3. It is not possible to call a *cofunction* without a ``cocall`` keyword. 4. ``cocall`` grammatically requires parentheses after it:: atom: cocall | <existing alternatives for atom> cocall: 'cocall' atom cotrailer* '(' [arglist] ')' cotrailer: '[' subscriptlist ']' | '.' NAME 5. ``cocall f(*args, **kwds)`` is semantically equivalent to ``yield from f.__cocall__(*args, **kwds)``. Differences from this proposal: 1. There is no equivalent of ``__cocall__`` in this PEP, which is called and its result is passed to ``yield from`` in the ``cocall`` expression. ``await`` keyword expects an *awaitable* object, validates the type, and executes ``yield from`` on it. Although, ``__await__`` method is similar to ``__cocall__``, but is only used to define *Future-like* objects. 2. ``await`` is defined in almost the same way as ``yield from`` in the grammar (it is later enforced that ``await`` can only be inside ``async def``). It is possible to simply write ``await future``, whereas ``cocall`` always requires parentheses. 3. To make asyncio work with PEP 3152 it would be required to modify ``(a)asyncio.coroutine`` decorator to wrap all functions in an object with a ``__cocall__`` method, or to implement ``__cocall__`` on generators. To call *cofunctions* from existing generator-based coroutines it would be required to use ``costart(cofunc, *args, **kwargs)`` built-in. 4. Since it is impossible to call a *cofunction* without a ``cocall`` keyword, it automatically prevents the common mistake of forgetting to use ``yield from`` on generator-based coroutines. This proposal addresses this problem with a different approach, see `Debugging Features`_. 5. A shortcoming of requiring a ``cocall`` keyword to call a coroutine is that if is decided to implement coroutine-generators -- coroutines with ``yield`` or ``async yield`` expressions -- we wouldn't need a ``cocall`` keyword to call them. So we'll end up having ``__cocall__`` and no ``__call__`` for regular coroutines, and having ``__call__`` and no ``__cocall__`` for coroutine- generators. 6. Requiring parentheses grammatically also introduces a whole lot of new problems. The following code:: await fut await function_returning_future() await asyncio.gather(coro1(arg1, arg2), coro2(arg1, arg2)) would look like:: cocall fut() # or cocall costart(fut) cocall (function_returning_future())() cocall asyncio.gather(costart(coro1, arg1, arg2), costart(coro2, arg1, arg2)) 7. There are no equivalents of ``async for`` and ``async with`` in PEP 3152. Coroutine-generators -------------------- With ``async for`` keyword it is desirable to have a concept of a *coroutine-generator* -- a coroutine with ``yield`` and ``yield from`` expressions. To avoid any ambiguity with regular generators, we would likely require to have an ``async`` keyword before ``yield``, and ``async yield from`` would raise a ``StopAsyncIteration`` exception. While it is possible to implement coroutine-generators, we believe that they are out of scope of this proposal. It is an advanced concept that should be carefully considered and balanced, with a non-trivial changes in the implementation of current generator objects. This is a matter for a separate PEP. Why "async" and "await" keywords -------------------------------- async/await is not a new concept in programming languages: * C# has it since long time ago [5]_; * proposal to add async/await in ECMAScript 7 [2]_; see also Traceur project [9]_; * Facebook's Hack/HHVM [6]_; * Google's Dart language [7]_; * Scala [8]_; * proposal to add async/await to C++ [10]_; * and many other less popular languages. This is a huge benefit, as some users already have experience with async/await, and because it makes working with many languages in one project easier (Python with ECMAScript 7 for instance). Why "__aiter__" returns awaitable --------------------------------- In principle, ``__aiter__`` could be a regular function. There are several good reasons to make it a coroutine: * as most of the ``__anext__``, ``__aenter__``, and ``__aexit__`` methods are coroutines, users would often make a mistake defining it as ``async`` anyways; * there might be a need to run some asynchronous operations in ``__aiter__``, for instance to prepare DB queries or do some file operation. Importance of "async" keyword ----------------------------- While it is possible to just implement ``await`` expression and treat all functions with at least one ``await`` as coroutines, this approach makes APIs design, code refactoring and its long time support harder. Let's pretend that Python only has ``await`` keyword:: def useful(): ... await log(...) ... def important(): await useful() If ``useful()`` function is refactored and someone removes all ``await`` expressions from it, it would become a regular python function, and all code that depends on it, including ``important()`` would be broken. To mitigate this issue a decorator similar to ``(a)asyncio.coroutine`` has to be introduced. Why "async def" --------------- For some people bare ``async name(): pass`` syntax might look more appealing than ``async def name(): pass``. It is certainly easier to type. But on the other hand, it breaks the symmetry between ``async def``, ``async with`` and ``async for``, where ``async`` is a modifier, stating that the statement is asynchronous. It is also more consistent with the existing grammar. Why not "await for" and "await with" ------------------------------------ ``async`` is an adjective, and hence it is a better choice for a *statement qualifier* keyword. ``await for/with`` would imply that something is awaiting for a completion of a ``for`` or ``with`` statement. Why "async def" and not "def async" ----------------------------------- ``async`` keyword is a *statement qualifier*. A good analogy to it are "static", "public", "unsafe" keywords from other languages. "async for" is an asynchronous "for" statement, "async with" is an asynchronous "with" statement, "async def" is an asynchronous function. Having "async" after the main statement keyword might introduce some confusion, like "for async item in iterator" can be read as "for each asynchronous item in iterator". Having ``async`` keyword before ``def``, ``with`` and ``for`` also makes the language grammar simpler. And "async def" better separates coroutines from regular functions visually. Why not a __future__ import --------------------------- ``__future__`` imports are inconvenient and easy to forget to add. Also, they are enabled for the whole source file. Consider that there is a big project with a popular module named "async.py". With future imports it is required to either import it using ``__import__()`` or ``importlib.import_module()`` calls, or to rename the module. The proposed approach makes it possible to continue using old code and modules without a hassle, while coming up with a migration plan for future python versions. Why magic methods start with "a" -------------------------------- New asynchronous magic methods ``__aiter__``, ``__anext__``, ``__aenter__``, and ``__aexit__`` all start with the same prefix "a". An alternative proposal is to use "async" prefix, so that ``__aiter__`` becomes ``__async_iter__``. However, to align new magic methods with the existing ones, such as ``__radd__`` and ``__iadd__`` it was decided to use a shorter version. Why not reuse existing magic names ---------------------------------- An alternative idea about new asynchronous iterators and context managers was to reuse existing magic methods, by adding an ``async`` keyword to their declarations:: class CM: async def __enter__(self): # instead of __aenter__ ... This approach has the following downsides: * it would not be possible to create an object that works in both ``with`` and ``async with`` statements; * it would break backwards compatibility, as nothing prohibits from returning a Future-like objects from ``__enter__`` and/or ``__exit__`` in Python <= 3.4; * one of the main points of this proposal is to make native coroutines as simple and foolproof as possible, hence the clear separation of the protocols. Why not reuse existing "for" and "with" statements -------------------------------------------------- The vision behind existing generator-based coroutines and this proposal is to make it easy for users to see where the code might be suspended. Making existing "for" and "with" statements to recognize asynchronous iterators and context managers will inevitably create implicit suspend points, making it harder to reason about the code. Comprehensions -------------- Syntax for asynchronous comprehensions could be provided, but this construct is outside of the scope of this PEP. Async lambda functions ---------------------- Syntax for asynchronous lambda functions could be provided, but this construct is outside of the scope of this PEP. Performance =========== Overall Impact -------------- This proposal introduces no observable performance impact. Here is an output of python's official set of benchmarks [4]_: :: python perf.py -r -b default ../cpython/python.exe ../cpython-aw/python.exe [skipped] Report on Darwin ysmac 14.3.0 Darwin Kernel Version 14.3.0: Mon Mar 23 11:59:05 PDT 2015; root:xnu-2782.20.48~5/RELEASE_X86_64 x86_64 i386 Total CPU cores: 8 ### etree_iterparse ### Min: 0.365359 -> 0.349168: 1.05x faster Avg: 0.396924 -> 0.379735: 1.05x faster Significant (t=9.71) Stddev: 0.01225 -> 0.01277: 1.0423x larger The following not significant results are hidden, use -v to show them: django_v2, 2to3, etree_generate, etree_parse, etree_process, fastpickle, fastunpickle, json_dump_v2, json_load, nbody, regex_v8, tornado_http. Tokenizer modifications ----------------------- There is no observable slowdown of parsing python files with the modified tokenizer: parsing of one 12Mb file (``Lib/test/test_binop.py`` repeated 1000 times) takes the same amount of time. async/await ----------- The following micro-benchmark was used to determine performance difference between "async" functions and generators:: import sys import time def binary(n): if n <= 0: return 1 l = yield from binary(n - 1) r = yield from binary(n - 1) return l + 1 + r async def abinary(n): if n <= 0: return 1 l = await abinary(n - 1) r = await abinary(n - 1) return l + 1 + r def timeit(gen, depth, repeat): t0 = time.time() for _ in range(repeat): list(gen(depth)) t1 = time.time() print('{}({}) * {}: total {:.3f}s'.format( gen.__name__, depth, repeat, t1-t0)) The result is that there is no observable performance difference. Minimum timing of 3 runs :: abinary(19) * 30: total 12.985s binary(19) * 30: total 12.953s Note that depth of 19 means 1,048,575 calls. Reference Implementation ======================== The reference implementation can be found here: [3]_. List of high-level changes and new protocols -------------------------------------------- 1. New syntax for defining coroutines: ``async def`` and new ``await`` keyword. 2. New ``__await__`` method for Future-like objects, and new ``tp_await`` slot in ``PyTypeObject``. 3. New syntax for asynchronous context managers: ``async with``. And associated protocol with ``__aenter__`` and ``__aexit__`` methods. 4. New syntax for asynchronous iteration: ``async for``. And associated protocol with ``__aiter__``, ``__aexit__`` and new built- in exception ``StopAsyncIteration``. 5. New AST nodes: ``AsyncFunctionDef``, ``AsyncFor``, ``AsyncWith``, ``Await``. 6. New functions: ``sys.set_coroutine_wrapper(callback)``, ``sys.get_coroutine_wrapper()``, ``types.coroutine(gen)``, ``inspect.iscoroutinefunction()``, ``inspect.iscoroutine()``, and ``inspect.isawaitable()``. 7. New ``CO_COROUTINE`` and ``CO_NATIVE_COROUTINE`` bit flags for code objects. While the list of changes and new things is not short, it is important to understand, that most users will not use these features directly. It is intended to be used in frameworks and libraries to provide users with convenient to use and unambiguous APIs with ``async def``, ``await``, ``async for`` and ``async with`` syntax. Working example --------------- All concepts proposed in this PEP are implemented [3]_ and can be tested. :: import asyncio async def echo_server(): print('Serving on localhost:8000') await asyncio.start_server(handle_connection, 'localhost', 8000) async def handle_connection(reader, writer): print('New connection...') while True: data = await reader.read(8192) if not data: break print('Sending {:.10}... back'.format(repr(data))) writer.write(data) loop = asyncio.get_event_loop() loop.run_until_complete(echo_server()) try: loop.run_forever() finally: loop.close() References ========== .. [1] https://docs.python.org/3/library/asyncio-task.html#asyncio.coroutine .. [2] http://wiki.ecmascript.org/doku.php?id=strawman:async_functions .. [3] https://github.com/1st1/cpython/tree/await .. [4] https://hg.python.org/benchmarks .. [5] https://msdn.microsoft.com/en-us/library/hh191443.aspx .. [6] http://docs.hhvm.com/manual/en/hack.async.php .. [7] https://www.dartlang.org/articles/await-async/ .. [8] http://docs.scala-lang.org/sips/pending/async.html .. [9] https://github.com/google/traceur-compiler/wiki/LanguageFeatures#async-func… .. [10] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3722.pdf (PDF) .. [11] https://docs.python.org/3/reference/expressions.html#generator-iterator-met… .. [12] https://docs.python.org/3/reference/expressions.html#primaries Acknowledgments =============== I thank Guido van Rossum, Victor Stinner, Elvis Pranskevichus, Andrew Svetlov, and Łukasz Langa for their initial feedback. Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End:

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PEP 492: async/await in Python; v3
by Yury Selivanov May 6, 2015

May 6, 2015

Hi python-dev, Another round of updates. Reference implementation has been updated: https://github.com/1st1/cpython/tree/await (includes all things from the below summary of updates + tests). Summary: 1. "PyTypeObject.tp_await" slot. Replaces "tp_reserved". This is to enable implementation of Futures with C API. Must return an iterator if implemented. 2. New grammar for "await" expressions, see 'Syntax of "await" expression' section 3. inspect.iscoroutine() and inspect.iscoroutineobjects() functions. 4. Full separation of coroutines and generators. This is a big one; let's discuss. a) Coroutine objects raise TypeError (is NotImplementedError better?) in their __iter__ and __next__. Therefore it's not not possible to pass them to iter(), tuple(), next() and other similar functions that work with iterables. b) Because of (a), for..in iteration also does not work on coroutines anymore. c) 'yield from' only accept coroutine objects from generators decorated with 'types.coroutine'. That means that existing asyncio generator-based coroutines will happily yield from both coroutines and generators. *But* every generator-based coroutine *must* be decorated with `asyncio.coroutine()`. This is potentially a backwards incompatible change. d) inspect.isgenerator() and inspect.isgeneratorfunction() return `False` for coroutine objects & coroutine functions. e) Should we add a coroutine ABC (for cython etc)? I, personally, think this is highly necessary. First, separation of coroutines from generators is extremely important. One day there won't be generator-based coroutines, and we want to avoid any kind of confusion. Second, we only can do this in 3.5. This kind of semantics change won't be ever possible. asyncio recommends using @coroutine decorator, and most projects that I've seen do use it. Also there is no reason for people to use iter() and next() functions on coroutines when writing asyncio code. I doubt that this will cause serious backwards compatibility problems (asyncio also has provisional status). Thank you, Yury PEP: 492 Title: Coroutines with async and await syntax Version: $Revision$ Last-Modified: $Date$ Author: Yury Selivanov <yselivanov(a)sprymix.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 09-Apr-2015 Python-Version: 3.5 Post-History: 17-Apr-2015, 21-Apr-2015, 27-Apr-2015 Abstract ======== This PEP introduces new syntax for coroutines, asynchronous ``with`` statements and ``for`` loops. The main motivation behind this proposal is to streamline writing and maintaining asynchronous code, as well as to simplify previously hard to implement code patterns. Rationale and Goals =================== Current Python supports implementing coroutines via generators (PEP 342), further enhanced by the ``yield from`` syntax introduced in PEP 380. This approach has a number of shortcomings: * it is easy to confuse coroutines with regular generators, since they share the same syntax; async libraries often attempt to alleviate this by using decorators (e.g. ``(a)asyncio.coroutine`` [1]_); * it is not possible to natively define a coroutine which has no ``yield`` or ``yield from`` statements, again requiring the use of decorators to fix potential refactoring issues; * support for asynchronous calls is limited to expressions where ``yield`` is allowed syntactically, limiting the usefulness of syntactic features, such as ``with`` and ``for`` statements. This proposal makes coroutines a native Python language feature, and clearly separates them from generators. This removes generator/coroutine ambiguity, and makes it possible to reliably define coroutines without reliance on a specific library. This also enables linters and IDEs to improve static code analysis and refactoring. Native coroutines and the associated new syntax features make it possible to define context manager and iteration protocols in asynchronous terms. As shown later in this proposal, the new ``async with`` statement lets Python programs perform asynchronous calls when entering and exiting a runtime context, and the new ``async for`` statement makes it possible to perform asynchronous calls in iterators. Specification ============= This proposal introduces new syntax and semantics to enhance coroutine support in Python, it does not change the internal implementation of coroutines, which are still based on generators. It is strongly suggested that the reader understands how coroutines are implemented in Python (PEP 342 and PEP 380). It is also recommended to read PEP 3156 (asyncio framework) and PEP 3152 (Cofunctions). From this point in this document we use the word *coroutine* to refer to functions declared using the new syntax. *generator-based coroutine* is used where necessary to refer to coroutines that are based on generator syntax. New Coroutine Declaration Syntax -------------------------------- The following new syntax is used to declare a coroutine:: async def read_data(db): pass Key properties of coroutines: * ``async def`` functions are always coroutines, even if they do not contain ``await`` expressions. * It is a ``SyntaxError`` to have ``yield`` or ``yield from`` expressions in an ``async`` function. * Internally, a new code object flag - ``CO_COROUTINE`` - is introduced to enable runtime detection of coroutines (and migrating existing code). All coroutines have both ``CO_COROUTINE`` and ``CO_GENERATOR`` flags set. * Regular generators, when called, return a *generator object*; similarly, coroutines return a *coroutine object*. * ``StopIteration`` exceptions are not propagated out of coroutines, and are replaced with a ``RuntimeError``. For regular generators such behavior requires a future import (see PEP 479). types.coroutine() ----------------- A new function ``coroutine(gen)`` is added to the ``types`` module. It applies ``CO_COROUTINE`` flag to the passed generator-function's code object, making it to return a *coroutine object* when called. This feature enables an easy upgrade path for existing libraries. Await Expression ---------------- The following new ``await`` expression is used to obtain a result of coroutine execution:: async def read_data(db): data = await db.fetch('SELECT ...') ... ``await``, similarly to ``yield from``, suspends execution of ``read_data`` coroutine until ``db.fetch`` *awaitable* completes and returns the result data. It uses the ``yield from`` implementation with an extra step of validating its argument. ``await`` only accepts an *awaitable*, which can be one of: * A *coroutine object* returned from a *coroutine* or a generator decorated with ``types.coroutine()``. * An object with an ``__await__`` method returning an iterator. Any ``yield from`` chain of calls ends with a ``yield``. This is a fundamental mechanism of how *Futures* are implemented. Since, internally, coroutines are a special kind of generators, every ``await`` is suspended by a ``yield`` somewhere down the chain of ``await`` calls (please refer to PEP 3156 for a detailed explanation.) To enable this behavior for coroutines, a new magic method called ``__await__`` is added. In asyncio, for instance, to enable Future objects in ``await`` statements, the only change is to add ``__await__ = __iter__`` line to ``asyncio.Future`` class. Objects with ``__await__`` method are called *Future-like* objects in the rest of this PEP. Also, please note that ``__aiter__`` method (see its definition below) cannot be used for this purpose. It is a different protocol, and would be like using ``__iter__`` instead of ``__call__`` for regular callables. It is a ``TypeError`` if ``__await__`` returns anything but an iterator. * Objects defined with CPython C API with a ``tp_await`` function, returning an iterator (similar to ``__await__`` method). It is a ``SyntaxError`` to use ``await`` outside of a coroutine. It is a ``TypeError`` to pass anything other than an *awaitable* object to an ``await`` expression. Syntax of "await" expression '''''''''''''''''''''''''''' ``await`` keyword is defined differently from ``yield`` and ``yield from``. The main difference is that *await expressions* do not require parentheses around them most of the times. Examples:: ================================== ================================== Expression Will be parsed as ================================== ================================== ``if await fut: pass`` ``if (await fut): pass`` ``if await fut + 1: pass`` ``if (await fut) + 1: pass`` ``pair = await fut, 'spam'`` ``pair = (await fut), 'spam'`` ``with await fut, open(): pass`` ``with (await fut), open(): pass`` ``await foo()['spam'].baz()()`` ``await ( foo()['spam'].baz()() )`` ``return await coro()`` ``return ( await coro() )`` ``res = await coro() ** 2`` ``res = (await coro()) ** 2`` ``func(a1=await coro(), a2=0)`` ``func(a1=(await coro()), a2=0)`` ================================== ================================== See `Grammar Updates`_ section for details. Asynchronous Context Managers and "async with" ---------------------------------------------- An *asynchronous context manager* is a context manager that is able to suspend execution in its *enter* and *exit* methods. To make this possible, a new protocol for asynchronous context managers is proposed. Two new magic methods are added: ``__aenter__`` and ``__aexit__``. Both must return an *awaitable*. An example of an asynchronous context manager:: class AsyncContextManager: async def __aenter__(self): await log('entering context') async def __aexit__(self, exc_type, exc, tb): await log('exiting context') New Syntax '''''''''' A new statement for asynchronous context managers is proposed:: async with EXPR as VAR: BLOCK which is semantically equivalent to:: mgr = (EXPR) aexit = type(mgr).__aexit__ aenter = type(mgr).__aenter__(mgr) exc = True try: try: VAR = await aenter BLOCK except: exc = False exit_res = await aexit(mgr, *sys.exc_info()) if not exit_res: raise finally: if exc: await aexit(mgr, None, None, None) As with regular ``with`` statements, it is possible to specify multiple context managers in a single ``async with`` statement. It is an error to pass a regular context manager without ``__aenter__`` and ``__aexit__`` methods to ``async with``. It is a ``SyntaxError`` to use ``async with`` outside of a coroutine. Example ''''''' With asynchronous context managers it is easy to implement proper database transaction managers for coroutines:: async def commit(session, data): ... async with session.transaction(): ... await session.update(data) ... Code that needs locking also looks lighter:: async with lock: ... instead of:: with (yield from lock): ... Asynchronous Iterators and "async for" -------------------------------------- An *asynchronous iterable* is able to call asynchronous code in its *iter* implementation, and *asynchronous iterator* can call asynchronous code in its *next* method. To support asynchronous iteration: 1. An object must implement an ``__aiter__`` method returning an *awaitable* resulting in an *asynchronous iterator object*. 2. An *asynchronous iterator object* must implement an ``__anext__`` method returning an *awaitable*. 3. To stop iteration ``__anext__`` must raise a ``StopAsyncIteration`` exception. An example of asynchronous iterable:: class AsyncIterable: async def __aiter__(self): return self async def __anext__(self): data = await self.fetch_data() if data: return data else: raise StopAsyncIteration async def fetch_data(self): ... New Syntax '''''''''' A new statement for iterating through asynchronous iterators is proposed:: async for TARGET in ITER: BLOCK else: BLOCK2 which is semantically equivalent to:: iter = (ITER) iter = await type(iter).__aiter__(iter) running = True while running: try: TARGET = await type(iter).__anext__(iter) except StopAsyncIteration: running = False else: BLOCK else: BLOCK2 It is a ``TypeError`` to pass a regular iterable without ``__aiter__`` method to ``async for``. It is a ``SyntaxError`` to use ``async for`` outside of a coroutine. As for with regular ``for`` statement, ``async for`` has an optional ``else`` clause. Example 1 ''''''''' With asynchronous iteration protocol it is possible to asynchronously buffer data during iteration:: async for data in cursor: ... Where ``cursor`` is an asynchronous iterator that prefetches ``N`` rows of data from a database after every ``N`` iterations. The following code illustrates new asynchronous iteration protocol:: class Cursor: def __init__(self): self.buffer = collections.deque() def _prefetch(self): ... async def __aiter__(self): return self async def __anext__(self): if not self.buffer: self.buffer = await self._prefetch() if not self.buffer: raise StopAsyncIteration return self.buffer.popleft() then the ``Cursor`` class can be used as follows:: async for row in Cursor(): print(row) which would be equivalent to the following code:: i = await Cursor().__aiter__() while True: try: row = await i.__anext__() except StopAsyncIteration: break else: print(row) Example 2 ''''''''' The following is a utility class that transforms a regular iterable to an asynchronous one. While this is not a very useful thing to do, the code illustrates the relationship between regular and asynchronous iterators. :: class AsyncIteratorWrapper: def __init__(self, obj): self._it = iter(obj) async def __aiter__(self): return self async def __anext__(self): try: value = next(self._it) except StopIteration: raise StopAsyncIteration return value async for letter in AsyncIteratorWrapper("abc"): print(letter) Why StopAsyncIteration? ''''''''''''''''''''''' Coroutines are still based on generators internally. So, before PEP 479, there was no fundamental difference between :: def g1(): yield from fut return 'spam' and :: def g2(): yield from fut raise StopIteration('spam') And since PEP 479 is accepted and enabled by default for coroutines, the following example will have its ``StopIteration`` wrapped into a ``RuntimeError`` :: async def a1(): await fut raise StopIteration('spam') The only way to tell the outside code that the iteration has ended is to raise something other than ``StopIteration``. Therefore, a new built-in exception class ``StopAsyncIteration`` was added. Moreover, with semantics from PEP 479, all ``StopIteration`` exceptions raised in coroutines are wrapped in ``RuntimeError``. Debugging Features ------------------ One of the most frequent mistakes that people make when using generators as coroutines is forgetting to use ``yield from``:: @asyncio.coroutine def useful(): asyncio.sleep(1) # this will do noting without 'yield from' For debugging this kind of mistakes there is a special debug mode in asyncio, in which ``@coroutine`` decorator wraps all functions with a special object with a destructor logging a warning. Whenever a wrapped generator gets garbage collected, a detailed logging message is generated with information about where exactly the decorator function was defined, stack trace of where it was collected, etc. Wrapper object also provides a convenient ``__repr__`` function with detailed information about the generator. The only problem is how to enable these debug capabilities. Since debug facilities should be a no-op in production mode, ``@coroutine`` decorator makes the decision of whether to wrap or not to wrap based on an OS environment variable ``PYTHONASYNCIODEBUG``. This way it is possible to run asyncio programs with asyncio's own functions instrumented. ``EventLoop.set_debug``, a different debug facility, has no impact on ``@coroutine`` decorator's behavior. With this proposal, coroutines is a native, distinct from generators, concept. New methods ``set_coroutine_wrapper`` and ``get_coroutine_wrapper`` are added to the ``sys`` module, with which frameworks can provide advanced debugging facilities. It is also important to make coroutines as fast and efficient as possible, therefore there are no debug features enabled by default. Example:: async def debug_me(): await asyncio.sleep(1) def async_debug_wrap(generator): return asyncio.CoroWrapper(generator) sys.set_coroutine_wrapper(async_debug_wrap) debug_me() # <- this line will likely GC the coroutine object and # trigger asyncio.CoroWrapper's code. assert isinstance(debug_me(), asyncio.CoroWrapper) sys.set_coroutine_wrapper(None) # <- this unsets any # previously set wrapper assert not isinstance(debug_me(), asyncio.CoroWrapper) If ``sys.set_coroutine_wrapper()`` is called twice, the new wrapper replaces the previous wrapper. ``sys.set_coroutine_wrapper(None)`` unsets the wrapper. inspect.iscoroutine() and inspect.iscoroutineobject() ----------------------------------------------------- Two new functions are added to the ``inspect`` module: * ``inspect.iscoroutine(obj)`` returns ``True`` if ``obj`` is a coroutine object. * ``inspect.iscoroutinefunction(obj)`` returns ``True`` is ``obj`` is a coroutine function. Differences between coroutines and generators --------------------------------------------- A great effort has been made to make sure that coroutines and generators are separate concepts: 1. Coroutine objects do not implement ``__iter__`` and ``__next__`` methods. Therefore they cannot be iterated over or passed to ``iter()``, ``list()``, ``tuple()`` and other built-ins. They also cannot be used in a ``for..in`` loop. 2. ``yield from`` does not accept coroutine objects (unless it is used in a generator-based coroutine decorated with ``types.coroutine``.) 3. ``yield from`` does not accept coroutine objects from plain Python generators (*not* generator-based coroutines.) 4. ``inspect.isgenerator()`` and ``inspect.isgeneratorfunction()`` return ``False`` for coroutine objects and coroutine functions. Coroutine objects ----------------- Coroutines are based on generators internally, thus they share the implementation. Similarly to generator objects, coroutine objects have ``throw``, ``send`` and ``close`` methods. ``StopIteration`` and ``GeneratorExit`` play the same role for coroutine objects (although PEP 479 is enabled by default for coroutines). Glossary ======== :Coroutine: A coroutine function, or just "coroutine", is declared with ``async def``. It uses ``await`` and ``return value``; see `New Coroutine Declaration Syntax`_ for details. :Coroutine object: Returned from a coroutine function. See `Await Expression`_ for details. :Future-like object: An object with an ``__await__`` method, or a C object with ``tp_await`` function, returning an iterator. Can be consumed by an ``await`` expression in a coroutine. A coroutine waiting for a Future-like object is suspended until the Future-like object's ``__await__`` completes, and returns the result. See `Await Expression`_ for details. :Awaitable: A *Future-like* object or a *coroutine object*. See `Await Expression`_ for details. :Generator-based coroutine: Coroutines based in generator syntax. Most common example is ``(a)asyncio.coroutine``. :Asynchronous context manager: An asynchronous context manager has ``__aenter__`` and ``__aexit__`` methods and can be used with ``async with``. See `Asynchronous Context Managers and "async with"`_ for details. :Asynchronous iterable: An object with an ``__aiter__`` method, which must return an *asynchronous iterator* object. Can be used with ``async for``. See `Asynchronous Iterators and "async for"`_ for details. :Asynchronous iterator: An asynchronous iterator has an ``__anext__`` method. See `Asynchronous Iterators and "async for"`_ for details. List of functions and methods ============================= ================= =================================== ================= Method Can contain Can't contain ================= =================================== ================= async def func await, return value yield, yield from async def __a*__ await, return value yield, yield from def __a*__ return awaitable await def __await__ yield, yield from, return iterable await generator yield, yield from, return value await ================= =================================== ================= Where: * "async def func": coroutine; * "async def __a*__": ``__aiter__``, ``__anext__``, ``__aenter__``, ``__aexit__`` defined with the ``async`` keyword; * "def __a*__": ``__aiter__``, ``__anext__``, ``__aenter__``, ``__aexit__`` defined without the ``async`` keyword, must return an *awaitable*; * "def __await__": ``__await__`` method to implement *Future-like* objects; * generator: a "regular" generator, function defined with ``def`` and which contains a least one ``yield`` or ``yield from`` expression. Transition Plan =============== To avoid backwards compatibility issues with ``async`` and ``await`` keywords, it was decided to modify ``tokenizer.c`` in such a way, that it: * recognizes ``async def`` name tokens combination (start of a coroutine); * keeps track of regular functions and coroutines; * replaces ``'async'`` token with ``ASYNC`` and ``'await'`` token with ``AWAIT`` when in the process of yielding tokens for coroutines. This approach allows for seamless combination of new syntax features (all of them available only in ``async`` functions) with any existing code. An example of having "async def" and "async" attribute in one piece of code:: class Spam: async = 42 async def ham(): print(getattr(Spam, 'async')) # The coroutine can be executed and will print '42' Backwards Compatibility ----------------------- This proposal preserves 100% backwards compatibility. Grammar Updates --------------- Grammar changes are also fairly minimal:: decorated: decorators (classdef | funcdef | async_funcdef) async_funcdef: ASYNC funcdef compound_stmt: (if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt) async_stmt: ASYNC (funcdef | with_stmt | for_stmt) power: atom_expr ['**' factor] atom_expr: [AWAIT] atom trailer* Transition Period Shortcomings ------------------------------ There is just one. Until ``async`` and ``await`` are not proper keywords, it is not possible (or at least very hard) to fix ``tokenizer.c`` to recognize them on the **same line** with ``def`` keyword:: # async and await will always be parsed as variables async def outer(): # 1 def nested(a=(await fut)): pass async def foo(): return (await fut) # 2 Since ``await`` and ``async`` in such cases are parsed as ``NAME`` tokens, a ``SyntaxError`` will be raised. To workaround these issues, the above examples can be easily rewritten to a more readable form:: async def outer(): # 1 a_default = await fut def nested(a=a_default): pass async def foo(): # 2 return (await fut) This limitation will go away as soon as ``async`` and ``await`` ate proper keywords. Or if it's decided to use a future import for this PEP. Deprecation Plans ----------------- ``async`` and ``await`` names will be softly deprecated in CPython 3.5 and 3.6. In 3.7 we will transform them to proper keywords. Making ``async`` and ``await`` proper keywords before 3.7 might make it harder for people to port their code to Python 3. asyncio ------- ``asyncio`` module was adapted and tested to work with coroutines and new statements. Backwards compatibility is 100% preserved. The required changes are mainly: 1. Modify ``(a)asyncio.coroutine`` decorator to use new ``types.coroutine()`` function. 2. Add ``__await__ = __iter__`` line to ``asyncio.Future`` class. 3. Add ``ensure_task()`` as an alias for ``async()`` function. Deprecate ``async()`` function. Design Considerations ===================== PEP 3152 -------- PEP 3152 by Gregory Ewing proposes a different mechanism for coroutines (called "cofunctions"). Some key points: 1. A new keyword ``codef`` to declare a *cofunction*. *Cofunction* is always a generator, even if there is no ``cocall`` expressions inside it. Maps to ``async def`` in this proposal. 2. A new keyword ``cocall`` to call a *cofunction*. Can only be used inside a *cofunction*. Maps to ``await`` in this proposal (with some differences, see below.) 3. It is not possible to call a *cofunction* without a ``cocall`` keyword. 4. ``cocall`` grammatically requires parentheses after it:: atom: cocall | <existing alternatives for atom> cocall: 'cocall' atom cotrailer* '(' [arglist] ')' cotrailer: '[' subscriptlist ']' | '.' NAME 5. ``cocall f(*args, **kwds)`` is semantically equivalent to ``yield from f.__cocall__(*args, **kwds)``. Differences from this proposal: 1. There is no equivalent of ``__cocall__`` in this PEP, which is called and its result is passed to ``yield from`` in the ``cocall`` expression. ``await`` keyword expects an *awaitable* object, validates the type, and executes ``yield from`` on it. Although, ``__await__`` method is similar to ``__cocall__``, but is only used to define *Future-like* objects. 2. ``await`` is defined in almost the same way as ``yield from`` in the grammar (it is later enforced that ``await`` can only be inside ``async def``). It is possible to simply write ``await future``, whereas ``cocall`` always requires parentheses. 3. To make asyncio work with PEP 3152 it would be required to modify ``(a)asyncio.coroutine`` decorator to wrap all functions in an object with a ``__cocall__`` method, or to implement ``__cocall__`` on generators. To call *cofunctions* from existing generator-based coroutines it would be required to use ``costart(cofunc, *args, **kwargs)`` built-in. 4. Since it is impossible to call a *cofunction* without a ``cocall`` keyword, it automatically prevents the common mistake of forgetting to use ``yield from`` on generator-based coroutines. This proposal addresses this problem with a different approach, see `Debugging Features`_. 5. A shortcoming of requiring a ``cocall`` keyword to call a coroutine is that if is decided to implement coroutine-generators -- coroutines with ``yield`` or ``async yield`` expressions -- we wouldn't need a ``cocall`` keyword to call them. So we'll end up having ``__cocall__`` and no ``__call__`` for regular coroutines, and having ``__call__`` and no ``__cocall__`` for coroutine- generators. 6. Requiring parentheses grammatically also introduces a whole lot of new problems. The following code:: await fut await function_returning_future() await asyncio.gather(coro1(arg1, arg2), coro2(arg1, arg2)) would look like:: cocall fut() # or cocall costart(fut) cocall (function_returning_future())() cocall asyncio.gather(costart(coro1, arg1, arg2), costart(coro2, arg1, arg2)) 7. There are no equivalents of ``async for`` and ``async with`` in PEP 3152. Coroutine-generators -------------------- With ``async for`` keyword it is desirable to have a concept of a *coroutine-generator* -- a coroutine with ``yield`` and ``yield from`` expressions. To avoid any ambiguity with regular generators, we would likely require to have an ``async`` keyword before ``yield``, and ``async yield from`` would raise a ``StopAsyncIteration`` exception. While it is possible to implement coroutine-generators, we believe that they are out of scope of this proposal. It is an advanced concept that should be carefully considered and balanced, with a non-trivial changes in the implementation of current generator objects. This is a matter for a separate PEP. No implicit wrapping in Futures ------------------------------- There is a proposal to add similar mechanism to ECMAScript 7 [2]_. A key difference is that JavaScript "async functions" always return a Promise. While this approach has some advantages, it also implies that a new Promise object is created on each "async function" invocation. We could implement a similar functionality in Python, by wrapping all coroutines in a Future object, but this has the following disadvantages: 1. Performance. A new Future object would be instantiated on each coroutine call. Moreover, this makes implementation of ``await`` expressions slower (disabling optimizations of ``yield from``). 2. A new built-in ``Future`` object would need to be added. 3. Coming up with a generic ``Future`` interface that is usable for any use case in any framework is a very hard to solve problem. 4. It is not a feature that is used frequently, when most of the code is coroutines. Why "async" and "await" keywords -------------------------------- async/await is not a new concept in programming languages: * C# has it since long time ago [5]_; * proposal to add async/await in ECMAScript 7 [2]_; see also Traceur project [9]_; * Facebook's Hack/HHVM [6]_; * Google's Dart language [7]_; * Scala [8]_; * proposal to add async/await to C++ [10]_; * and many other less popular languages. This is a huge benefit, as some users already have experience with async/await, and because it makes working with many languages in one project easier (Python with ECMAScript 7 for instance). Why "__aiter__" is a coroutine ------------------------------ In principle, ``__aiter__`` could be a regular function. There are several good reasons to make it a coroutine: * as most of the ``__anext__``, ``__aenter__``, and ``__aexit__`` methods are coroutines, users would often make a mistake defining it as ``async`` anyways; * there might be a need to run some asynchronous operations in ``__aiter__``, for instance to prepare DB queries or do some file operation. Importance of "async" keyword ----------------------------- While it is possible to just implement ``await`` expression and treat all functions with at least one ``await`` as coroutines, this approach makes APIs design, code refactoring and its long time support harder. Let's pretend that Python only has ``await`` keyword:: def useful(): ... await log(...) ... def important(): await useful() If ``useful()`` function is refactored and someone removes all ``await`` expressions from it, it would become a regular python function, and all code that depends on it, including ``important()`` would be broken. To mitigate this issue a decorator similar to ``(a)asyncio.coroutine`` has to be introduced. Why "async def" --------------- For some people bare ``async name(): pass`` syntax might look more appealing than ``async def name(): pass``. It is certainly easier to type. But on the other hand, it breaks the symmetry between ``async def``, ``async with`` and ``async for``, where ``async`` is a modifier, stating that the statement is asynchronous. It is also more consistent with the existing grammar. Why "async for/with" instead of "await for/with" ------------------------------------------------ ``async`` is an adjective, and hence it is a better choice for a *statement qualifier* keyword. ``await for/with`` would imply that something is awaiting for a completion of a ``for`` or ``with`` statement. Why "async def" and not "def async" ----------------------------------- ``async`` keyword is a *statement qualifier*. A good analogy to it are "static", "public", "unsafe" keywords from other languages. "async for" is an asynchronous "for" statement, "async with" is an asynchronous "with" statement, "async def" is an asynchronous function. Having "async" after the main statement keyword might introduce some confusion, like "for async item in iterator" can be read as "for each asynchronous item in iterator". Having ``async`` keyword before ``def``, ``with`` and ``for`` also makes the language grammar simpler. And "async def" better separates coroutines from regular functions visually. Why not a __future__ import --------------------------- ``__future__`` imports are inconvenient and easy to forget to add. Also, they are enabled for the whole source file. Consider that there is a big project with a popular module named "async.py". With future imports it is required to either import it using ``__import__()`` or ``importlib.import_module()`` calls, or to rename the module. The proposed approach makes it possible to continue using old code and modules without a hassle, while coming up with a migration plan for future python versions. Why magic methods start with "a" -------------------------------- New asynchronous magic methods ``__aiter__``, ``__anext__``, ``__aenter__``, and ``__aexit__`` all start with the same prefix "a". An alternative proposal is to use "async" prefix, so that ``__aiter__`` becomes ``__async_iter__``. However, to align new magic methods with the existing ones, such as ``__radd__`` and ``__iadd__`` it was decided to use a shorter version. Why not reuse existing magic names ---------------------------------- An alternative idea about new asynchronous iterators and context managers was to reuse existing magic methods, by adding an ``async`` keyword to their declarations:: class CM: async def __enter__(self): # instead of __aenter__ ... This approach has the following downsides: * it would not be possible to create an object that works in both ``with`` and ``async with`` statements; * it would break backwards compatibility, as nothing prohibits from returning a Future-like objects from ``__enter__`` and/or ``__exit__`` in Python <= 3.4; * one of the main points of this proposal is to make coroutines as simple and foolproof as possible, hence the clear separation of the protocols. Why not reuse existing "for" and "with" statements -------------------------------------------------- The vision behind existing generator-based coroutines and this proposal is to make it easy for users to see where the code might be suspended. Making existing "for" and "with" statements to recognize asynchronous iterators and context managers will inevitably create implicit suspend points, making it harder to reason about the code. Comprehensions -------------- For the sake of restricting the broadness of this PEP there is no new syntax for asynchronous comprehensions. This should be considered in a separate PEP, if there is a strong demand for this feature. Async lambdas ------------- Lambda coroutines are not part of this proposal. In this proposal they would look like ``async lambda(parameters): expression``. Unless there is a strong demand to have them as part of this proposal, it is recommended to consider them later in a separate PEP. Performance =========== Overall Impact -------------- This proposal introduces no observable performance impact. Here is an output of python's official set of benchmarks [4]_: :: python perf.py -r -b default ../cpython/python.exe ../cpython-aw/python.exe [skipped] Report on Darwin ysmac 14.3.0 Darwin Kernel Version 14.3.0: Mon Mar 23 11:59:05 PDT 2015; root:xnu-2782.20.48~5/RELEASE_X86_64 x86_64 i386 Total CPU cores: 8 ### etree_iterparse ### Min: 0.365359 -> 0.349168: 1.05x faster Avg: 0.396924 -> 0.379735: 1.05x faster Significant (t=9.71) Stddev: 0.01225 -> 0.01277: 1.0423x larger The following not significant results are hidden, use -v to show them: django_v2, 2to3, etree_generate, etree_parse, etree_process, fastpickle, fastunpickle, json_dump_v2, json_load, nbody, regex_v8, tornado_http. Tokenizer modifications ----------------------- There is no observable slowdown of parsing python files with the modified tokenizer: parsing of one 12Mb file (``Lib/test/test_binop.py`` repeated 1000 times) takes the same amount of time. async/await ----------- The following micro-benchmark was used to determine performance difference between "async" functions and generators:: import sys import time def binary(n): if n <= 0: return 1 l = yield from binary(n - 1) r = yield from binary(n - 1) return l + 1 + r async def abinary(n): if n <= 0: return 1 l = await abinary(n - 1) r = await abinary(n - 1) return l + 1 + r def timeit(gen, depth, repeat): t0 = time.time() for _ in range(repeat): list(gen(depth)) t1 = time.time() print('{}({}) * {}: total {:.3f}s'.format( gen.__name__, depth, repeat, t1-t0)) The result is that there is no observable performance difference. Minimum timing of 3 runs :: abinary(19) * 30: total 12.985s binary(19) * 30: total 12.953s Note that depth of 19 means 1,048,575 calls. Reference Implementation ======================== The reference implementation can be found here: [3]_. List of high-level changes and new protocols -------------------------------------------- 1. New syntax for defining coroutines: ``async def`` and new ``await`` keyword. 2. New ``__await__`` method for Future-like objects, and new ``tp_await`` slot in ``PyTypeObject``. 3. New syntax for asynchronous context managers: ``async with``. And associated protocol with ``__aenter__`` and ``__aexit__`` methods. 4. New syntax for asynchronous iteration: ``async for``. And associated protocol with ``__aiter__``, ``__aexit__`` and new built- in exception ``StopAsyncIteration``. 5. New AST nodes: ``AsyncFunctionDef``, ``AsyncFor``, ``AsyncWith``, ``Await``. 6. New functions: ``sys.set_coroutine_wrapper(callback)``, ``sys.get_coroutine_wrapper()``, ``types.coroutine(gen)``, ``inspect.iscoroutinefunction()``, and ``inspect.iscoroutine()``. 7. New ``CO_COROUTINE`` bit flag for code objects. While the list of changes and new things is not short, it is important to understand, that most users will not use these features directly. It is intended to be used in frameworks and libraries to provide users with convenient to use and unambiguous APIs with ``async def``, ``await``, ``async for`` and ``async with`` syntax. Working example --------------- All concepts proposed in this PEP are implemented [3]_ and can be tested. :: import asyncio async def echo_server(): print('Serving on localhost:8000') await asyncio.start_server(handle_connection, 'localhost', 8000) async def handle_connection(reader, writer): print('New connection...') while True: data = await reader.read(8192) if not data: break print('Sending {:.10}... back'.format(repr(data))) writer.write(data) loop = asyncio.get_event_loop() loop.run_until_complete(echo_server()) try: loop.run_forever() finally: loop.close() References ========== .. [1] https://docs.python.org/3/library/asyncio-task.html#asyncio.coroutine .. [2] http://wiki.ecmascript.org/doku.php?id=strawman:async_functions .. [3] https://github.com/1st1/cpython/tree/await .. [4] https://hg.python.org/benchmarks .. [5] https://msdn.microsoft.com/en-us/library/hh191443.aspx .. [6] http://docs.hhvm.com/manual/en/hack.async.php .. [7] https://www.dartlang.org/articles/await-async/ .. [8] http://docs.scala-lang.org/sips/pending/async.html .. [9] https://github.com/google/traceur-compiler/wiki/LanguageFeatures#async-func… .. [10] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3722.pdf (PDF) Acknowledgments =============== I thank Guido van Rossum, Victor Stinner, Elvis Pranskevichus, Andrew Svetlov, and Łukasz Langa for their initial feedback. Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End:

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PEP 492 vs. PEP 3152, new round
by Guido van Rossum May 1, 2015

May 1, 2015

I've tried to catch up with the previous threads. A summary of issues brought up: 1. precise syntax of `async def` (or do we need it at all) 2. do we need `async for` and `async with` (and how to spell them) 3. syntactic priority of `await` 4. `cocall` vs. `await` 5. do we really need `__aiter__` and friends 6. StopAsyncException 7. compatibility with asyncio and existing users of it (I've added a few myself.) I'll try to take them one by one. *1. precise syntax of `async def`* Of all the places to put `async` I still like *before* the `def` the best. I often do "imprecise search" for e.g. /def foo/ and would be unhappy if this didn't find async defs. Putting it towards the end (`def foo async()` or `def foo() async`) makes it easier to miss. A decorator makes it hard to make the syntactic distinctions required to reject `await` outside an async function. So I still prefer *`async def`*. *2. do we need `async for` and `async with`* Yes we do. Most of you are too young to remember, but once upon a time you couldn't loop over the lines of a file with a `for` loop like you do now. The amount of code that was devoted to efficiently iterate over files was tremendous. We're back in that stone age with the asyncio `StreamReader` class -- it supports `read()`, `readline()` and so on, but you can't use it with `for`, so you have to write a `while True` loop. `asyncio for` makes it possible to add a simple `__anext__` to the `StreamReader` class, as follows: ``` async def __anext__(self): line = await self.readline() if not line: raise StopAsyncIteration return line ``` A similar argument can be made for `async with`; the transaction commit is pretty convincing, but it also helps to be able to wait e.g. for a transport to drain upon closing a write stream. As for how to spell these, I think having `async` at the front makes it most clear that this is a special form. (Though maybe we should consider `await for` and `await with`? That would have the advantage of making it easy to scan for all suspension points by searching for /await/. But being a verb it doesn't read very well.) *3. syntactic priority of `await`* Yury, could you tweak the syntax for `await` so that we can write the most common usages without parentheses? In particular I'd like to be able to write ``` return await foo() with await foo() as bar: ... foo(await bar(), await bletch()) ``` (I don't care about `await foo() + await bar()` but it would be okay.) ``` I think this is reasonable with some tweaks of the grammar (similar to what Greg did for cocall, but without requiring call syntax at the end). *4. `cocall` vs. `await`* Python evolves. We couldn't have PEP 380 (`yield from`) without prior experience with using generators as coroutines (PEP 342), which in turn required basic generators (PEP 255), and those were a natural evolution of Python's earlier `for` loop. We couldn't PEP 3156 (asyncio) without PEP 380 and all that came before. The asyncio library is getting plenty of adoption and it has the concept of separating the *getting* of a future[1] from *waiting* for it. IIUC this is also how `await` works in C# (it just requires something with an async type). This has enabled a variety of operations that take futures and produce more futures. [1] I write `future` with a lowercase 'f' to include concepts like coroutine generator objects. *I just can't get used to this aspect of PEP 3152, so I'm rejecting it.* Sorry Greg, but that's the end. We must see `await` as a refinement of `yield from`, not as an alternative. (Yury: PEP 492 is not accepted yet, but you're getting closer.) One more thing: this separation is "Pythonic" in the sense that it's similar to the way *getting* a callable object is a separate act from *calling* it. While this is a cause for newbie bugs (forgetting to call an argument-less function) it has also enabled the concept of "callable" as more general and more powerful in Python: any time you need to pass a callable, you can pass e.g. a bound method or a class or something you got from `functools.partial`, and that's a useful thing (other languages require you to introduce something like a lambda in such cases, which can be painful if the thing you wrap has a complex signature -- or they don't support function parameters at all, like Java). I know that Greg defends it by explaining that `cocal f(args)` is not a `cocall` operator applied to `f(args)`, it is the *single* operator `cocall ...(args)` applied to `f`. But this is too subtle, and it just doesn't jive with the long tradition of using `yield from f` where f is some previously obtained future. *5. do we really need `__aiter__` and friends* There's a lot of added complexity, but I think it's worth it. I don't think we need to make the names longer, the 'a' prefix is fine for these methods. I think it's all in the protocols: regular `with` uses `__enter__` and `__exit__`; `async with` uses `__aenter__` and `__aexit__` (which must return futures). Ditto for `__aiter__` and `__anext__`. I guess this means that the async equivalent to obtaining an iterator through `it = iter(xs)` followed by `for x over it` will have to look like `ait = await aiter(xs)` followed by `for x over ait`, where an iterator is required to have an `__aiter__` method that's an async function and returns self immediately. But what if you left out the `await` from the first call? I.e. can this work? ``` ait = aiter(xs) async for x in ait: print(x) ``` The question here is whether the object returned by aiter(xs) has an `__aiter__` method. Since it was intended to be the target of `await`, it has an `__await__` method. But that itself is mostly an alias for `__iter__`, not `__aiter__`. I guess it can be made to work, the object just has to implement a bunch of different protocols. *6. StopAsyncException* I'm not sure about this. The motivation given in the PEP seems to focus on the need for `__anext__` to be async. But is this really the right pattern? What if we required `ait.__anext__()` to return a future, which can either raise good old `StopIteration` or return the next value from the iteration when awaited? I'm wondering if there are a few alternatives to be explored around the async iterator protocol still. *7. compatibility with asyncio and existing users of it* This is just something I want to stress. On the one hand it should be really simple to take code written for pre-3.5 asyncio and rewrite it to PEP 492 -- simple change `(a)asyncio.coroutine` to `async def` and change `yield from` to `await`. (Everything else is optional; existing patterns for loops and context managers should continue to work unchanged, even if in some cases you may be able to simplify the code by using `async for` and `async with`.) But it's also important that *even if you don't switch* (i.e. if you want to keep your code compatible with pre-3.5 asyncio) you can still use the PEP 492 version of asyncio -- i.e. the asyncio library that comes with 3.5 must seamlessly support mixing code that uses `await` and code that uses `yield from`. And this should go both ways -- if you have some code that uses PEP 492 and some code that uses pre-3.5 asyncio, they should be able to pass their coroutines to each other and wait for each other's coroutines. That's all I have for now. Enjoy! -- --Guido van Rossum (python.org/~guido)

14 70 0 0

Python-Dev

April 2015