In December 2021, this PEP and PEP 667 converged on a common definition of the proposed changes to the Python level semantics of the locals() builtin (as documented in the PEP text below), with the only remaining differences being in the proposed C API changes and various internal implementation details.
Of those remaining differences, the most significant one was that PEP 667 at the time still proposed an immediate backwards compatibility break for the PyEval_GetLocals() API as soon as the PEP was accepted and implemented.
PEP 667 has since been changed to propose a generous deprecation period for the PyEval_GetLocals() API, continuing to support it in parallel with the improved semantics offered by the new PyEval_GetFrameLocals() API.
Any remaining C API design concerns relate to new informational APIs that can be added at a later date if they are deemed necessary, and any potential concerns about the exact performance characteristics of the frame locals view implementation are outweighed by the availability of a viable reference implementation.
Accordingly, this PEP has been withdrawn in favour of proceeding with PEP 667.
Note: while implementing PEP 667 it became apparent that the rationale for and impact of locals() being updated to return independent snapshots in optimized scopes was not entirely clear in either PEP. The Motivation and Rationale sections in this PEP have been updated accordingly (since those aspects are equally applicable to the accepted PEP 667).
The semantics of the locals() builtin have historically been underspecified and hence implementation dependent.
This PEP proposes formally standardising on the behaviour of the CPython 3.10 reference implementation for most execution scopes, with some adjustments to the behaviour at function scope to make it more predictable and independent of the presence or absence of tracing functions.
In addition, it proposes that the following functions be added to the stable Python C API/ABI:
It also proposes the addition of several supporting functions and type definitions to the CPython C API.
While the precise semantics of the locals() builtin are nominally undefined, in practice, many Python programs depend on it behaving exactly as it behaves in CPython (at least when no tracing functions are installed).
Other implementations such as PyPy are currently replicating that behaviour, up to and including replication of local variable mutation bugs that can arise when a trace hook is installed [1].
While this PEP considers CPython’s current behaviour when no trace hooks are installed to be largely acceptable, it considers the current behaviour when trace hooks are installed to be problematic, as it causes bugs like [1] without even reliably enabling the desired functionality of allowing debuggers like pdb to mutate local variables [3].
Review of the initial PEP and the draft implementation then identified an opportunity for simplification of both the documentation and implementation of the function level locals() behaviour by updating it to return an independent snapshot of the function locals and closure variables on each call, rather than continuing to return the semi-dynamic intermittently updated shared copy that it has historically returned in CPython.
Specifically, the proposal in this PEP eliminates the historical behaviour where adding a new local variable can change the behaviour of code executed with exec() in function scopes, even if that code runs before the local variable is defined.
For example:
prints 1, but:
prints None (the default value from the .get() call).
With this PEP both examples would print None, as the call to exec() and the subsequent call to locals() would use independent dictionary snapshots of the local variables rather than using the same shared dictionary cached on the frame object.
The expected semantics of the locals() builtin change based on the current execution scope. For this purpose, the defined scopes of execution are:
This PEP proposes elevating most of the current behaviour of the CPython reference implementation to become part of the language specification, except that each call to locals() at function scope will create a new dictionary object, rather than caching a common dict instance in the frame object that each invocation will update and return.
This PEP also proposes to largely eliminate the concept of a separate “tracing” mode from the CPython reference implementation. In releases up to and including Python 3.10, the CPython interpreter behaves differently when a trace hook has been registered in one or more threads via an implementation dependent mechanism like sys.settrace ([4]) in CPython’s sys module or PyEval_SetTrace ([5]) in CPython’s C API. If this PEP is accepted, then the only remaining behavioural difference when a trace hook is installed is that some optimisations in the interpreter eval loop are disabled when the tracing logic needs to run after each opcode.
This PEP proposes changes to CPython’s behaviour at function scope that make the locals() builtin semantics when a trace hook is registered identical to those used when no trace hook is registered, while also making the related frame API semantics clearer and easier for interactive debuggers to rely on.
The proposed elimination of tracing mode affects the semantics of frame object references obtained through other means, such as via a traceback, or via the sys._getframe() API, as the write-through semantics needed for trace hook support are always provided by the f_locals attribute on frame objects, rather than being runtime state dependent.
The heart of this proposal is to revise the documentation for the locals() builtin to read as follows:
At module scope, as well as when using exec() or eval() with a single namespace, this function returns the same namespace as globals().
At class scope, it returns the namespace that will be passed to the metaclass constructor.
When using exec() or eval() with separate local and global namespaces, it returns the local namespace passed in to the function call.
In all of the above cases, each call to locals() in a given frame of execution will return the same mapping object. Changes made through the mapping object returned from locals() will be visible as bound, rebound, or deleted local variables, and binding, rebinding, or deleting local variables will immediately affect the contents of the returned mapping object.
At function scope (including for generators and coroutines), each call to locals() instead returns a fresh dictionary containing the current bindings of the function’s local variables and any nonlocal cell references. In this case, name binding changes made via the returned dict are not written back to the corresponding local variables or nonlocal cell references, and binding, rebinding, or deleting local variables and nonlocal cell references does not affect the contents of previously returned dictionaries.
There would also be a versionchanged note for the release making this change:
For reference, the current documentation of this builtin reads as follows:
Note: The contents of this dictionary should not be modified; changes may not affect the values of local and free variables used by the interpreter.
(In other words: the status quo is that the semantics and behaviour of locals() are formally implementation defined, whereas the proposed state after this PEP is that the only implementation defined behaviour will be that associated with whether or not the implementation emulates the CPython frame API, with the behaviour in all other cases being defined by the language and library references)
At module scope, as well as when using exec() or eval() with a single namespace, locals() must return the same object as globals(), which must be the actual execution namespace (available as inspect.currentframe().f_locals in implementations that provide access to frame objects).
Variable assignments during subsequent code execution in the same scope must dynamically change the contents of the returned mapping, and changes to the returned mapping must change the values bound to local variable names in the execution environment.
To capture this expectation as part of the language specification, the following paragraph will be added to the documentation for locals():
This part of the proposal does not require any changes to the reference implementation - it is standardisation of the current behaviour.
At class scope, as well as when using exec() or eval() with separate global and local namespaces, locals() must return the specified local namespace (which may be supplied by the metaclass __prepare__ method in the case of classes). As for module scope, this must be a direct reference to the actual execution namespace (available as inspect.currentframe().f_locals in implementations that provide access to frame objects).
Variable assignments during subsequent code execution in the same scope must change the contents of the returned mapping, and changes to the returned mapping must change the values bound to local variable names in the execution environment.
The mapping returned by locals() will not be used as the actual class namespace underlying the defined class (the class creation process will copy the contents to a fresh dictionary that is only accessible by going through the class machinery).
For nested classes defined inside a function, any nonlocal cells referenced from the class scope are not included in the locals() mapping.
To capture this expectation as part of the language specification, the following two paragraphs will be added to the documentation for locals():
At class scope, it returns the namespace that will be passed to the metaclass constructor.
This part of the proposal does not require any changes to the reference implementation - it is standardisation of the current behaviour.
At function scope, interpreter implementations are granted significant freedom to optimise local variable access, and hence are NOT required to permit arbitrary modification of local and nonlocal variable bindings through the mapping returned from locals().
Historically, this leniency has been described in the language specification with the words “The contents of this dictionary should not be modified; changes may not affect the values of local and free variables used by the interpreter.”
This PEP proposes to change that text to instead say:
This part of the proposal does require changes to the CPython reference implementation, as CPython currently returns a shared mapping object that may be implicitly refreshed by additional calls to locals(), and the “write back” strategy currently used to support namespace changes from trace functions also doesn’t comply with it (and causes the quirky behavioural problems mentioned in the Motivation above).
The implementation of the locals() builtin is modified to return a distinct copy of the local namespace for optimised frames, rather than a direct reference to the internal frame value cache updated by the PyFrame_FastToLocals() C API and returned by the PyEval_GetLocals() C API.
The current cause of CPython’s tracing mode quirks (both the side effects from simply installing a tracing function and the fact that writing values back to function locals only works for the specific function being traced) is the way that locals mutation support for trace hooks is currently implemented: the PyFrame_LocalsToFast function.
When a trace function is installed, CPython currently does the following for function frames (those where the code object uses “fast locals” semantics):
This approach is problematic for a few different reasons:
The proposed resolution to this problem is to take advantage of the fact that whereas functions typically access their own namespace using the language defined locals() builtin, trace functions necessarily use the implementation dependent frame.f_locals interface, as a frame reference is what gets passed to hook implementations.
Instead of being a direct reference to the internal frame value cache historically returned by the locals() builtin, the Python level frame.f_locals will be updated to instead return instances of a dedicated fast locals proxy type that writes and reads values directly to and from the fast locals array on the underlying frame. Each access of the attribute produces a new instance of the proxy (so creating proxy instances is intentionally a cheap operation).
Despite the new proxy type becoming the preferred way to access local variables on optimised frames, the internal value cache stored on the frame is still retained for two key purposes:
With the changes in this PEP, this internal frame value cache is no longer directly accessible from Python code (whereas historically it was both returned by the locals() builtin and available as the frame.f_locals attribute). Instead, the value cache is only accessible via the PyEval_GetLocals() C API and by directly accessing the internal storage of a frame object.
Fast locals proxy objects and the internal frame value cache returned by PyEval_GetLocals() offer the following behavioural guarantees:
As a result of these points, only code using PyEval_GetLocals(), PyLocals_Get(), or PyLocals_GetCopy() will need to be concerned about the frame value cache potentially becoming stale. Code using the new frame fast locals proxy API (whether from Python or from C) will always see the live state of the frame.
Each fast locals proxy instance has a single internal attribute that is not exposed as part of the Python runtime API:
In addition, proxy instances use and update the following attributes stored on the underlying frame or code object:
__getitem__ operations on the proxy will populate the _name_to_offset_mapping on the code object (if it is not already populated), and then either return the relevant value (if the key is found in either the _name_to_offset_mapping mapping or the internal frame value cache), or else raise KeyError. Variables that are defined on the frame but not currently bound also raise KeyError (just as they’re omitted from the result of locals()).
As the frame storage is always accessed directly, the proxy will automatically pick up name binding and unbinding operations that take place as the function executes. The internal value cache is implicitly updated when individual variables are read from the frame state (including for containment checks, which need to check if the name is currently bound or unbound).
Similarly, __setitem__ and __delitem__ operations on the proxy will directly affect the corresponding fast local or cell reference on the underlying frame, ensuring that changes are immediately visible to the running Python code, rather than needing to be written back to the runtime storage at some later time. Such changes are also immediately written to the internal frame value cache to make them visible to users of the PyEval_GetLocals() C API.
Keys that are not defined as local or closure variables on the underlying frame are still written to the internal value cache on optimised frames. This allows utilities like pdb (which writes __return__ and __exception__ values into the frame’s f_locals mapping) to continue working as they always have. These additional keys that do not correspond to a local or closure variable on the frame will be left alone by future cache sync operations. Using the frame value cache to store these extra keys (rather than defining a new mapping that holds only the extra keys) provides full interoperability with the existing PyEval_GetLocals() API (since users of either API will see extra keys added by users of either API, rather than users of the new fast locals proxy API only seeing keys added via that API).
An additional benefit of storing only the variable value cache on the frame (rather than storing an instance of the proxy type), is that it avoids creating a reference cycle from the frame back to itself, so the frame will only be kept alive if another object retains a reference to a proxy instance.
Note: calling the proxy.clear() method has a similarly broad impact as calling PyFrame_LocalsToFast() on an empty frame value cache in earlier versions. Not only will the frame local variables be cleared, but also any cell variables accessible from the frame (whether those cells are owned by the frame itself or by an outer frame). This can clear a class’s __class__ cell if called on the frame of a method that uses the zero-arg super() construct (or otherwise references __class__). This exceeds the scope of calling frame.clear(), as that only drop’s the frame’s references to cell variables, it doesn’t clear the cells themselves. This PEP could be a potential opportunity to narrow the scope of attempts to clear the frame variables directly by leaving cells belonging to outer frames alone, and only clearing local variables and cells belonging directly to the frame underlying the proxy (this issue affects PEP 667 as well, as the question relates to the handling of cell variables, and is entirely independent of the internal frame value cache).
Unlike Python code, extension module functions that call in to the Python C API can be called from any kind of Python scope. This means it isn’t obvious from the context whether locals() will return a snapshot or not, as it depends on the scope of the calling Python code, not the C code itself.
This means it is desirable to offer C APIs that give predictable, scope independent, behaviour. However, it is also desirable to allow C code to exactly mimic the behaviour of Python code at the same scope.
To enable mimicking the behaviour of Python code, the stable C ABI would gain the following new functions:
PyLocals_Get() is directly equivalent to the Python locals() builtin. It returns a new reference to the local namespace mapping for the active Python frame at module and class scope, and when using exec() or eval(). It returns a shallow copy of the active namespace at function/coroutine/generator scope.
PyLocals_GetKind() returns a value from the newly defined PyLocals_Kind enum, with the following options being available:
Since the enum is used in the stable ABI, an additional 31-bit value is set to ensure that it is safe to cast arbitrary signed 32-bit signed integers to PyLocals_Kind values.
This query API allows extension module code to determine the potential impact of mutating the mapping returned by PyLocals_Get() without needing access to the details of the running frame object. Python code gets equivalent information visually through lexical scoping (as covered in the new locals() builtin documentation).
To allow extension module code to behave consistently regardless of the active Python scope, the stable C ABI would gain the following new function:
PyLocals_GetCopy() returns a new dict instance populated from the current locals namespace. Roughly equivalent to dict(locals()) in Python code, but avoids the double-copy in the case where locals() already returns a shallow copy. Akin to the following code, but doesn’t assume there will only ever be two kinds of locals result:
The existing PyEval_GetLocals() API will retain its existing behaviour in CPython (mutable locals at class and module scope, shared dynamic snapshot otherwise). However, its documentation will be updated to note that the conditions under which the shared dynamic snapshot get updated have changed.
The PyEval_GetLocals() documentation will also be updated to recommend replacing usage of this API with whichever of the new APIs is most appropriate for the use case:
The existing PyEval_GetLocals() API returns a borrowed reference, which means it cannot be updated to return the new shallow copies at function scope. Instead, it will continue to return a borrowed reference to an internal dynamic snapshot stored on the frame object. This shared mapping will behave similarly to the existing shared mapping in Python 3.10 and earlier, but the exact conditions under which it gets refreshed will be different. Specifically, it will be updated only in the following circumstance:
Requesting a fast locals proxy will not implicitly update the shared dynamic snapshot, and the CPython trace hook handling will no longer implicitly update it either.
(Note: even though PyEval_GetLocals() is part of the stable C API/ABI, the specifics of when the namespace it returns gets refreshed are still an interpreter implementation detail)
The additions to the public CPython C API are the frame level enhancements needed to support the stable C API/ABI updates:
PyFrame_GetLocalsKind(frame) is the underlying API for PyLocals_GetKind().
PyFrame_GetLocals(frame) is the underlying API for PyLocals_Get().
PyFrame_GetLocalsCopy(frame) is the underlying API for PyLocals_GetCopy().
_PyFrame_BorrowLocals(frame) is the underlying API for PyEval_GetLocals(). The underscore prefix is intended to discourage use and to indicate that code using it is unlikely to be portable across implementations. However, it is documented and visible to the linker in order to avoid having to access the internals of the frame struct from the PyEval_GetLocals() implementation.
The PyFrame_LocalsToFast() function will be changed to always emit RuntimeError, explaining that it is no longer a supported operation, and affected code should be updated to use PyObject_GetAttrString(frame, "f_locals") to obtain a read/write proxy instead.
In addition to the above documented interfaces, the draft reference implementation also exposes the following undocumented interfaces:
This type is what the reference implementation actually returns from PyObject_GetAttrString(frame, "f_locals") for optimized frames (i.e. when PyFrame_GetLocalsKind() returns PyLocals_SHALLOW_COPY).
As noted in [9], the implicit call to PyFrame_FastToLocals() in the Python trace hook support isn’t free, and could be rendered unnecessary if the frame proxy read values directly from the frame instead of getting them from the mapping.
As the new frame locals proxy type doesn’t require separate data refresh steps, this PEP incorporates Victor Stinner’s proposal to no longer implicitly call PyFrame_FastToLocalsWithError() before calling trace hooks implemented in Python.
Code using the new fast locals proxy objects will have the dynamic locals snapshot implicitly refreshed when accessing methods that need it, while code using the PyEval_GetLocals() API will implicitly refresh it when making that call.
The PEP necessarily also drops the implicit call to PyFrame_LocalsToFast() when returning from a trace hook, as that API now always raises an exception.
The locals() builtin is a required part of the language, and in the reference implementation it has historically returned a mutable mapping with the following characteristics:
Originally this PEP proposed to retain the first two of these properties, while changing the third in order to address the outright behaviour bugs that it can cause.
In [7] Nathaniel Smith made a persuasive case that we could make the behaviour of locals() at function scope substantially less confusing by retaining only the second property and having each call to locals() at function scope return an independent snapshot of the local variables and closure references rather than updating an implicitly shared snapshot.
As this revised design also made the implementation markedly easier to follow, the PEP was updated to propose this change in behaviour, rather than retaining the historical shared snapshot.
As discussed in [7], it would theoretically be possible to change the semantics of the locals() builtin to return the write-through proxy at function scope, rather than switching it to return independent snapshots.
This PEP doesn’t (and won’t) propose this as it’s a backwards incompatible change in practice, even though code that relies on the current behaviour is technically operating in an undefined area of the language specification.
Consider the following code snippet:
Even with a trace hook installed, that function will consistently print 1 on the current reference interpreter implementation:
Similarly, locals() can be passed to the exec() and eval() builtins at function scope (either explicitly or implicitly) without risking unexpected rebinding of local variables or closure references.
Provoking the reference interpreter into incorrectly mutating the local variable state requires a more complex setup where a nested function closes over a variable being rebound in the outer function, and due to the use of either threads, generators, or coroutines, it’s possible for a trace function to start running for the nested function before the rebinding operation in the outer function, but finish running after the rebinding operation has taken place (in which case the rebinding will be reverted, which is the bug reported in [1]).
In addition to preserving the de facto semantics which have been in place since PEP 227 introduced nested scopes in Python 2.1, the other benefit of restricting the write-through proxy support to the implementation-defined frame object API is that it means that only interpreter implementations which emulate the full frame API need to offer the write-through capability at all, and that JIT-compiled implementations only need to enable it when a frame introspection API is invoked, or a trace hook is installed, not whenever locals() is accessed at function scope.
Returning snapshots from locals() at function scope also means that static analysis for function level code will be more reliable, as only access to the frame machinery will allow rebinding of local and nonlocal variable references in a way that is hidden from static analysis.
These are formally defined as inheriting globals() and locals() from the calling scope by default.
There isn’t any need for the PEP to change these defaults, so it doesn’t, and exec() and eval() will start running in a shallow copy of the local namespace when that is what locals() returns.
This behaviour will have potential performance implications, especially for functions with large numbers of local variables (e.g. if these functions are called in a loop, calling globals() and locals() once before the loop and then passing the namespace into the function explicitly will give the same semantics and performance characteristics as the status quo, whereas relying on the implicit default would create a new shallow copy of the local namespace on each iteration).
(Note: the reference implementation draft PR has updated the locals() and vars(), eval(), and exec() builtins to use PyLocals_Get(). The dir() builtin still uses PyEval_GetLocals(), since it’s only using it to make a list from the keys).
Note: while implementing PEP 667, it was noted that neither that PEP nor this one clearly explained the impact the locals() changes would have on code execution APIs like exec() and eval(). This section was added to this PEP’s rationale to better describe the impact and explain the intended benefits of the change.
When exec() was converted from a statement to a builtin function in Python 3.0 (part of the core language changes in PEP 3100), the associated implicit call to PyFrame_LocalsToFast() was removed, so it typically appears as if attempts to write to local variables with exec() in optimized frames are ignored:
In truth, the writes aren’t being ignored, they just aren’t being copied from the dictionary cache back to the optimized local variable array. The changes to the dictionary are then overwritten the next time the dictionary cache is refreshed from the array:
The behaviour becomes even stranger if a tracing function or another piece of code invokes PyFrame_LocalsToFast() before the cache is next refreshed. In those cases the change is written back to the optimized local variable array:
This situation was more common in Python 3.10 and earlier versions, as merely installing a tracing function was enough to trigger implicit calls to PyFrame_LocalsToFast() after every line of Python code. However, it can still happen in Python 3.11+ depending on exactly which tracing functions are active (e.g. interactive debuggers intentionally do this so that changes made at the debugging prompt are visible when code execution resumes).
All of the above comments in relation to exec() apply to any attempt to mutate the result of locals() in optimized scopes, and are the main reason that the locals() builtin docs contain this caveat:
While the exact wording in the library reference is not entirely explicit, both exec() and eval() have long used the results of calling globals() and locals() in the calling Python frame as their default execution namespace.
This was historically also equivalent to using the calling frame’s frame.f_globals and frame.f_locals attributes, but this PEP maps the default namespace arguments for exec() and eval() to globals() and locals() in the calling frame in order to preserve the property of defaulting to ignoring attempted writes to the local namespace in optimized scopes.
This poses a potential compatibility issue for some code, as with the previous implementation that returns the same dict when locals() is called multiple times in function scope, the following code usually worked due to the implicitly shared local variable namespace:
With locals() in an optimised scope returning the same shared dict for each call, it was possible to store extra “fake locals” in that dict. While these aren’t real locals known by the compiler (so they can’t be printed with code like print(a)), they can still be accessed via locals() and shared between multiple exec() calls in the same function scope. Furthermore, because they’re not real locals, they don’t get implicitly updated or removed when the shared cache is refreshed from the local variable storage array.
When the code in exec() tries to write to an existing local variable, the runtime behaviour gets harder to predict:
print(a) will print None because the implicit locals() call in exec() refreshes the cached dict with the actual values on the frame. This means that, unlike the “fake” locals created by writing back to locals() (including via previous calls to exec()), the real locals known by the compiler can’t easily be modified by exec() (it can be done, but it requires both retrieving the frame.f_locals attribute to enable writes back to the frame, and then invoking PyFrame_LocalsToFast(), as shown using ctypes above).
As noted in the Motivation section, this confusing side effect happens even if the local variable is only defined after the exec() calls:
Because a is a real local variable that is not currently bound to a value, it gets explicitly removed from the dictionary returned by locals() whenever locals() is called prior to the a = None line. This removal is intentional, as it allows the contents of locals() to be updated correctly in optimized scopes when del statements are used to delete previously bound local variables.
As noted in the ctypes example, the above behavioural description may be invalidated if the CPython PyFrame_LocalsToFast() API gets invoked while the frame is still running. In that case, the changes to a might become visible to the running code, depending on exactly when that API is called (and whether the frame has been primed for locals modification by accessing the frame.f_locals attribute).
As described above, two options were considered to replace this confusing behaviour:
The PEP chooses the second option for the following reasons:
With the semantic changes to locals() in this PEP, it becomes much easier to explain the behavior of exec() and eval(): in optimized scopes, they will never implicitly affect local variables; in other scopes, they will always implicitly affect local variables. In optimized scopes, any implicit assignment to the local variables will be discarded when the code execution API returns, since a fresh copy of the local variables is used on each invocation.
Retaining the internal frame value cache results in some visible quirks when frame proxy instances are kept around and re-used after name binding and unbinding operations have been executed on the frame.
The primary reason for retaining the frame value cache is to maintain backwards compatibility with the PyEval_GetLocals() API. That API returns a borrowed reference, so it must refer to persistent state stored on the frame object. Storing a fast locals proxy object on the frame creates a problematic reference cycle, so the cleanest option is to instead continue to return a frame value cache, just as this function has done since optimised frames were first introduced.
With the frame value cache being kept around anyway, it then further made sense to rely on it to simplify the fast locals proxy mapping implementation.
Note: the fact PEP 667 doesn’t use the internal frame value cache as part of the write-through proxy implementation is the key Python level difference between the two PEPs.
Note: when this PEP was first written, it predated the Python 3.11 change to drop the implicit writeback of the frame local variables whenever a tracing function was installed, so making that change was included as part of the proposal.
Earlier versions of this PEP proposed having the semantics of the frame f_locals attribute depend on whether or not a tracing hook was currently installed - only providing the write-through proxy behaviour when a tracing hook was active, and otherwise behaving the same as the historical locals() builtin.
That was adopted as the original design proposal for a couple of key reasons, one pragmatic and one more philosophical:
However, actually attempting to implement and document that dynamic approach highlighted the fact that it makes for a really subtle runtime state dependent behaviour distinction in how frame.f_locals works, and creates several new edge cases around how f_locals behaves as trace functions are added and removed.
Accordingly, the design was switched to the current one, where frame.f_locals is always a write-through proxy, and locals() is always a snapshot, which is both simpler to implement and easier to explain.
Regardless of how the CPython reference implementation chooses to handle this, optimising compilers and interpreters also remain free to impose additional restrictions on debuggers, such as making local variable mutation through frame objects an opt-in behaviour that may disable some optimisations (just as the emulation of CPython’s frame API is already an opt-in flag in some Python implementations).
One of the draft iterations of this PEP proposed removing the ability to store additional data on optimised frames by writing to frame.f_locals keys that didn’t correspond to local or closure variable names on the underlying frame.
While this idea offered some attractive simplification of the fast locals proxy implementation, pdb stores __return__ and __exception__ values on arbitrary frames, so the standard library test suite fails if that functionality no longer works.
Accordingly, the ability to store arbitrary keys was retained, at the expense of certain operations on proxy objects being slower than could otherwise be (since they can’t assume that only names defined on the code object will be accessible through the proxy).
It is expected that the exact details of the interaction between the fast locals proxy and the f_locals value cache on the underlying frame will evolve over time as opportunities for improvement are identified.
The current semantics of mutating locals() and frame.f_locals in CPython are rather quirky due to historical implementation details:
This proposal deliberately doesn’t formalise these semantics as is, since they only make sense in terms of the historical evolution of the language and the reference implementation, rather than being deliberately designed.
Historically, the CPython C API (and subsequently, the stable ABI) has exposed only a single API function related to the Python locals builtin: PyEval_GetLocals(). However, as it returns a borrowed reference, it is not possible to adapt that interface directly to supporting the new locals() semantics proposed in this PEP.
An earlier iteration of this PEP proposed a minimalist adaptation to the new semantics: one C API function that behaved like the Python locals() builtin, and another that behaved like the frame.f_locals descriptor (creating and returning the write-through proxy if necessary).
The feedback [8] on that version of the C API was that it was too heavily based on how the Python level semantics were implemented, and didn’t account for the behaviours that authors of C extensions were likely to need.
The broader API now being proposed came from grouping the potential reasons for wanting to access the Python locals() namespace from an extension module into the following cases:
Historically, these kinds of checks and operations would only have been possible if a Python implementation emulated the full CPython frame API. With the proposed API, extension modules can instead ask more clearly for the semantics that they actually need, giving Python implementations more flexibility in how they provide those capabilities.
NOTE: the comparison below is against PEP 667 as it was in December 2021. It does not reflect the state of PEP 667 as of April 2024 (when this PEP was withdrawn in favour of proceeding with PEP 667).
PEP 667 offers a partially competing proposal for this PEP that suggests it would be reasonable to eliminate the internal frame value cache on optimised frames entirely.
These changes were originally offered as amendments to PEP 558, and the PEP author rejected them for three main reasons:
Of the three reasons, the first is the most important (since we need compelling reasons to break API backwards compatibility, and we don’t have them).
However, after reviewing PEP 667’s proposed Python level semantics, the author of this PEP eventually agreed that they would be simpler for users of the Python locals() API, so this distinction between the two PEPs has been eliminated: regardless of which PEP and implementation is accepted, the fast locals proxy object always provides a consistent view of the current state of the local variables, even if this results in some operations becoming O(n) that would be O(1) on a regular dictionary (specifically, len(proxy) becomes O(n), since it needs to check which names are currently bound, and proxy mapping comparisons avoid relying on the length check optimisation that allows differences in the number of stored keys to be detected quickly for regular mappings).
Due to the adoption of these non-standard performance characteristics in the proxy implementation, the PyLocals_GetView() and PyFrame_GetLocalsView() C APIs were also removed from the proposal in this PEP.
This leaves the only remaining points of distinction between the two PEPs as specifically related to the C API:
The Python pseudo-code below is based on the implementation sketch presented in PEP 667 as of the time of writing (2021-10-24). The differences that provide the improved interoperability between the new fast locals proxy API and the existing PyEval_GetLocals() API are noted in comments.
As in PEP 667, all attributes that start with an underscore are invisible and cannot be accessed directly. They serve only to illustrate the proposed design.
For simplicity (and as in PEP 667), the handling of module and class level frames is omitted (they’re much simpler, as _locals is the execution namespace, so no translation is required).
Note: the simplest way to convert the earlier iterations of the PEP 558 reference implementation into a preliminary implementation of the now proposed semantics is to remove the frame_cache_updated checks in affected operations, and instead always sync the frame cache in those methods. Adopting that approach changes the algorithmic complexity of the following operations as shown (where n is the number of local and cell variables defined on the frame):
The length check and value comparison operations have relatively limited opportunities for improvement: without allowing usage of a potentially stale cache, the only way to know how many variables are currently bound is to iterate over all of them and check, and if the implementation is going to be spending that many cycles on an operation anyway, it may as well spend it updating the frame value cache and then consuming the result. These operations are O(n) in both this PEP and in PEP 667. Customised implementations could be provided that are faster than updating the frame cache, but it’s far from clear that the extra code complexity needed to speed these operations up would be worthwhile when it only offers a linear performance improvement rather than an algorithmic complexity improvement.
The O(1) nature of the other operations can be restored by adding implementation code that doesn’t rely on the value cache being up to date.
Keeping the iterator/iterable retrieval methods as O(1) will involve writing custom replacements for the corresponding builtin dict helper types, just as proposed in PEP 667. As illustrated above, the implementations would be similar to the pseudo-code presented in PEP 667, but not identical (due to the improved PyEval_GetLocals() interoperability offered by this PEP affecting the way it stores extra variables).
popitem() can be improved from “always O(n)” to “O(n) worst case” by creating a custom implementation that relies on the improved iteration APIs.
To ensure stale frame information is never presented in the Python fast locals proxy API, these changes in the reference implementation will need to be implemented before merging.
The current implementation at time of writing (2021-10-24) also still stores a copy of the fast refs mapping on each frame rather than storing a single instance on the underlying code object (as it still stores cell references directly, rather than check for cells on each fast locals array access). Fixing this would also be required before merging.
The reference implementation update is in development as a draft pull request on GitHub ([6]).
Thanks to Nathaniel J. Smith for proposing the write-through proxy idea in [1] and pointing out some critical design flaws in earlier iterations of the PEP that attempted to avoid introducing such a proxy.
Thanks to Steve Dower and Petr Viktorin for asking that more attention be paid to the developer experience of the proposed C API additions [8] [13].
Thanks to Larry Hastings for the suggestion on how to use enums in the stable ABI while ensuring that they safely support typecasting from arbitrary integers.
Thanks to Mark Shannon for pushing for further simplification of the C level API and semantics, as well as significant clarification of the PEP text (and for restarting discussion on the PEP in early 2021 after a further year of inactivity) [10] [11] [12]. Mark’s comments that were ultimately published as PEP 667 also directly resulted in several implementation efficiency improvements that avoid incurring the cost of redundant O(n) mapping refresh operations when the relevant mappings aren’t used, as well as the change to ensure that the state reported through the Python level f_locals API is never stale.
This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.
Source: https://github.com/python/peps/blob/main/peps/pep-0558.rst
Last modified: 2025-02-01 08:55:40 UTC