PEP 508 specifies a mini-language for declaring package dependencies. One feature of this language is the ability to specify extras, which are optional components of a distribution that, when used, install additional dependencies. This PEP proposes a mechanism to allow one or more extras to be installed by default if none are provided explicitly.
Various use cases for default extras and possible solutions in this PEP were discussed extensively on this DPO thread. These fall into two broad cases that provide the motivation for the present PEP.
Package maintainers often use extras to declare optional dependencies that extend the functionality or performance of a package. In some cases, it can be difficult to determine which dependencies should be required and which should be categorized as extras. A balance must be struck between the needs of typical users, who may prefer most features to be available by default, and users who want minimal installations without large, optional dependencies. One solution with existing Python packaging infrastructure is for package maintainers to define an extra called, for example, recommended, which includes all non-essential but suggested dependencies. Users are then instructed to install the package using package[recommended], while those who prefer more control can use package. However, in practice, many users are unaware of the [recommended] syntax, placing the burden on them to know this for a typical installation. Having a way to have recommended dependencies be installed by default while providing a way for users to request a more minimal installation would satisfy this use case, and this PEP describes a solution to this.
Examples of packages that demonstrate this pattern by encouraging users to include extra dependencies by default include:
Another common use case for using extras is to define different backends or frontends and dependencies that need to be installed for each backend or frontend. A package might need at least one backend or frontend to be installed in order to be functional, but may be flexible on which backend or frontend this is. Concrete examples of such frontends or backends include:
With current packaging standards, maintainers have to either require one of the backends or frontends or require users to always specify extras, e.g., package[backend], and therefore risk users having an unusable installation if they only install package. Having a way to specify one or more default backend or frontend and providing a way to override these defaults would provide a much better experience for users, and the approach described in this PEP will allow this.
Note that this PEP does not aim to address the issue of disallowing conflicting or incompatible extras, for example if a package requires exactly one frontend or backend package. There is currently no mechanism in Python packaging infrastructure to disallow conflicting or incompatible extras to be installed, and this PEP does not change that.
Examples of packages that require at least one backend or frontend to work and recommend a default extra to install a backend or frontend include:
In all three cases, installing the package without any extras results in a broken installation, and this is a commonly reported support issue for some of these packages.
A number of possible solutions have been discussed extensively by the community for several years, including in this DPO thread as well as in numerous issues and pull requests. The solution that is presented below:
It is the only solution, out of all those discussed, that meets all three criteria.
A new multiple-use metadata field, Default-Extra, will be added to the core package metadata. For this field, each entry must be a string specifying an extra that will be automatically included when the package is installed without any extras being specified explicitly.
Only entries already specified in a Provides-Extra entry can be used in a Default-Extra entry.
Examples:
Since this introduces a new field in the core package metadata, this will require Metadata-Version to be bumped to the next minor version (2.5 at the time of writing).
A new key will be added to the [project] table in project metadata as originally defined in PEP 621 and now defined in the PyPA specifications. This key will be named default-optional-dependency-keys with the following description:
Each string in default-optional-dependency-keys must be the name of an extra defined in optional-dependencies, and each extra in this array will be converted to a matching Default-Extra entry in the core package metadata. Examples of valid usage which would produce the example Default-Extra entries presented in the previous section are:
and:
If extras are explicitly given in a dependency specification, the default extras are ignored. Otherwise, the default extras are installed.
For example, if a package defines an extra1 default extra as well as a non-default extra2 extra, then if a user were to install the package with:
the default extra1 dependency would be included. If the user instead installs the package with:
then the extra2 extra would be installed but the default extra1 extra would be ignored.
If the same package is specified multiple times in an installation command or dependency tree, the default extras must be installed if any of the instances of the package are specified without extras. For instance, if one installs a package spam where package appears several times in the dependency tree:
then the default extra should be installed because package appears at least once with no extras specified.
An empty set of extras, such as package[] should be interpreted as meaning that the package should be installed without any default extras (unless package appears elsewhere in the dependency tree, in which case, the default extra would be installed as mentioned above). This would provide a universal way of obtaining a minimal installation of a package.
We also note that some tools such as pip currently ignore unrecognized extras, and emit a warning to the user to indicate that the extra has not been recognized, e.g:
For tools that behave like this (rather than raising an error), if an extra is recognized as invalid in a dependency specification, it should be ignored, and if all specified extras are invalid, then this should be considered equivalent to package[] (rather than package) and not install any default extras.
Finally, we note (as also discussed in Relying on tooling to deselect any default extras) that package installers are allowed to implement their own options to control the above behavior, for example implementing an option that disables default extras for some or all packages regardless of where these packages appear in the dependency tree. If such tool-specific options are implemented, tool developers should make these opt-in, and users should experience the above PEP 771 behavior as default.
In this section we take a look at the use cases described in the Motivation section and how these can now be addressed by using the specification outlined above.
First, we consider the case of packages that want recommended but not strictly required dependencies installed by default, while also providing a way to only install the required dependencies.
In order to do this, a package maintainer would define an extra called recommended containing the recommended but not required dependencies, and would choose to have this be included as a default extra:
If this package was called package, users installing package would then get the equivalent of package[recommended]. Users could alternatively install package[] which would install the package without the default extras.
To take a one of the concrete examples of package from the Motivation section, the astropy package defines a recommended extra that users are currently instructed to install in the default installation instructions. With this PEP, the recommended extra could be declared as being a default extra:
meaning that installing:
would then install optional but recommended dependencies such as scipy. Advanced users who want a minimal install could then use:
As described in Motivation, some packages may support multiple backends and/or frontends, and in some cases it may be desirable to ensure that there is always at least one backend or frontend package installed, as the package would be unusable otherwise. Concrete examples of this might include a GUI application that needs a GUI library to be present to be usable but is able to support different ones, or a package that can rely on different computational backends but needs at least one to be installed.
In this case, package maintainers could make the choice to define an extra for each backend or frontend, and provide a default, e.g.:
If packages can support e.g. multiple backends at the same time, and some of the backends should always be installed, then the dependencies for these must be given as required dependencies rather than using the default extras mechanism.
To take one of the concrete examples mentioned in Motivation, the napari package can make use of one of PyQt5, PyQt6, PySide2, or PySide6, and users currently need to explicitly specify napari[all] in order to have one of these be installed, or e.g., napari[pyqt5] to explicitly specify one of the frontend packages. Installing napari with no extras results in a non-functional package. With this PEP, napari could define the following configuration:
meaning that:
would work out-of-the-box, but there would still be a mechanism for users to explicitly specify a frontend, e.g.:
An additional case we consider here is where a package maintainer wants to support the ability for users to opt-in to non-default extras, without removing default extras. Essentially, they might want:
This could be achieved with e.g:
The ability for a package to reference itself in the extras is supported by existing Python packaging tools.
Once again considering a concrete example, astropy could with this PEP define a recommended extra (as described in Recommended dependencies and minimal installations). However, it also defines other extras, including for example jupyter, which adds packages that enhance the user experience inside Jupyter-based environments. It is possible that users opting in to this extra would still want recommended dependencies to be installed. In this case, the following configuration would solve this case:
Users installing:
would then get the same as:
In some cases, it may be that packages need multiple kinds of defaults. As an example, in Packages requiring at least one backend or frontend, we considered the case of packages that have either backends or frontends, but in some cases, packages may have to support backends and frontends, and want to specify one or more default frontend and one or more default backend.
Ideally, one may want the following behavior:
However, this PEP chooses not to provide a mechanism for making it so that e.g., if backend1 is specified, the default backend would be disabled, but the default frontend would be enabled, since this adds complexity.
Maintainers should instead for now document that if a backend or frontend is explicitly specified, both backend and frontend need to be specified. Discoverability for users who want to do this should not be an issue however since users need to read the documentation in any case to find out what backends or frontends are available, so they can be shown at the same time how to properly use the extras for backends and frontends.
One option to increase user friendliness is that maintainers can create extras called for example defaultbackend and defaultfrontend which do install the default backend and frontend. They can then recommend that users do:
This would allow (if desired) users to then get whatever the recommended backend is, even if that default changes in time.
If there is a desire to implement a better solution in future, we believe this PEP should not preclude this. For example, one could imagine in future adding the ability for an extra to specify which default extras it disables, and if this is not specified then explicitly specified extras would disable all default extras (consistent with the present PEP).
Once support for this PEP is added to tools in the packaging ecosystem, packages that do not make use of default extras will continue to work as-is and there should be no break in compatibility.
Once packages start defining default extras, those defaults will only be honored with recent versions of packaging tools which implement this PEP, but those packages will remain installable with older packaging tools – with the main difference being that the default extras will not be installed automatically when older packaging tools are used.
As described in How to teach this, package authors need to carefully evaluate when and how they adopt the default extra feature depending on their user base, as some actions (such as moving a required dependency to a default extra) will likely result in breakage for users if a significant fraction of them are still using older package installers that do not support default extras. In this sense, package authors should be aware that this feature, if used in certain ways, can cause backward-compatibility issues for users, and they are thus responsible for ensuring that they minimize the impact to users.
The most significant backward-compatibility aspect is related to assumptions packaging tools make about extras – specifically, this PEP changes the assumption that extras are no longer exclusively additive in terms of adding dependencies to the dependency tree, and specifying some extras can result in fewer dependencies being installed.
A specific example of change in behavior can be seen with pip: consider a package package which has a required dependency of numpy, and a default extra called recommended which includes scipy. If a user installs package[], only package and numpy will be installed. If a user then does:
then requirements.txt will contain e.g.:
However, if the user then installs the requirements from this file using:
then pip will install package (which will include the default extra) as well as numpy, so the final environment will contain scipy. A solution in this specific case is for the user to pass --no-deps to pip install to avoid resolving the dependency tree, but the point here is to illustrate that there may be changes in behavior in packaging tools due to the change in the assumption about what impact an extra can have.
It is worth noting that the recently-accepted PEP 751 defines a new file format which is intended to replace alternatives such as the pip freeze output and other tools in future. The new file format is designed so that the packages in the file are installed without resolving dependencies, which means that it will be fully compatible with default extras as specified in this PEP, and will avoid the issue with pip freeze/pip install -r mentioned above.
There are no known security implications for this PEP.
This section outlines information that should be made available to people in different groups in the community in relation to the implementation of this PEP. Some aspects described below will be relevant even before the PEP is fully implemented in packaging tools as there are some preparations that can be done in advance of this implementation to facilitate any potential transition later on. The groups covered below are:
Package users should be provided with clear installation instructions that show what extras are available for packages and how they behave, for example explaining that by default some recommended dependencies or a given frontend or backend will be installed, and how to opt out of this or override defaults, depending what is available.
While the mechanism used to define extras and the associated rule about when to use it are clear, package authors need to carefully consider several points before adopting this capability in their packages, to avoid inadvertently breaking backward-compatibility.
Package installers such as pip or uv will not necessarily implement support for default extras at the same time, and once they do it is likely that package authors will want to keep supporting users who do not have the most recent version of a package installer. In this case, the following recommendations would apply:
One temptation for authors might be to include many dependencies by default since they can provide a way to opt out from these. We recommend however that authors carefully consider what is included by default to avoid unnecessarily bloating installations and complicating dependency trees. Using default extras does not mean that all extras need to be defaults, and there is still scope for users to explicitly opt in to non-default extras.
Default extras should generally be treated with the same “weight” as required dependencies. When a package is widely used, introducing a default extra will result in that extra’s dependencies being transitively included – unless all downstream packages are updated to explicitly opt out using minimal installation specifications.
As an example, the pytest package currently has nearly 1,500 plugins that depend on it. If pytest were to add a default extra and those plugins were not updated accordingly, installing a plugin would include the default extras’ dependencies. This doesn’t preclude the use of default extras, but addition of default extras requires careful evaluation of its downstream effects.
If package authors choose to make an extra be installed by default, it is important that they are aware that if users explicitly specify another extra, the default may not be installed, unless they use the approach described in Supporting extras that should not remove default extras.
There are cases, such as the interchangeable backends, where ignoring the default if an extra is explicitly specified is the right thing to do. However, for other cases, such as using default extras to include recommended dependencies while still providing a way to do minimal installs, it may be that many of the other extras should explicitly ‘inherit’ the default extra(s), so package authors should carefully consider in which cases they want the default extras to be installed.
In some cases, it may be that packages have extras that are mutually incompatible. In this case, we recommend against using the default extra feature for any extra that contains a dependency that could be incompatible with another.
Consider a package that has extras package[A] and package[B]. Users could already currently try and install package[A] and package[B] or package[A,B] which would result in a broken installation, however it would at least be explicit that both extras were being installed. Making A into a default extra however could lead to unintuitive issues. A user could do:
and end up with a broken installation, even though A and B were never explicitly both installed. For this reason, we recommend against using default extras for dependencies where this is likely to be an issue.
Authors need to take special care when circular dependencies are present. For instance, consider the following dependency tree:
If package1 has a default extra named recommended then:
will still result in the recommended extra being installed if package2 continues to depend on package1 (with no extras specified). This could be solved by changing the dependency tree to instead be:
assuming that indeed package2 does not depend on any features provided by the extra dependencies of package1. Authors therefore need to carefully consider a migration plan, coordinating with the authors of package2.
Regardless of how default extras are used, package authors should aim to ensure that their package’s documentation makes it clear how extras are to be used. ‘Best practices’ documentation should mention:
The impact on individuals who repackage Python libraries for different distributions, such as conda, Homebrew, linux package installers (such as apt and yum) and so on, needs to be considered. Not all package distributions have mechanisms that would line up with the approach described. In fact, some distributions such as conda, do not even have the concept of extras.
There are two cases to consider here:
It is impossible for a PEP such as this to exhaustively consider each of the different package distributions. However, ultimately, default extras should be understood as how package authors would like their package to be installed for the majority of users, and this should inform decisions about how default extras should be handled, whether manually or automatically.
The following repository contains a fully functional demo package that makes use of default extras:
https://github.com/wheel-next/pep_771
This makes use of modified branches of several packages, and the following links are to these branches:
In addition, this branch contains a modified version of the Flit package.
The implementations above are proofs-of-concept at this time and the existing changes have not yet been reviewed by the relevant maintainers. Nevertheless, they are functional enough to allow for interested maintainers to try these out.
Using existing packaging tools and infrastructure, package maintainers who want to provide a minimal installation for some users and a default non-minimal installation for regular users (e.g. with recommended dependencies or a default backend) can technically already achieve this if they are willing to distribute two packages instead of one – for example package-core which would be the main package with minimal dependencies, and package which would be a metapackage that would depend on package-core with optional dependencies enabled.
Taking once again a concrete example from the Motivation section, the astropy package defines a recommended extra that users are currently instructed to install in the default installation instructions. In principle, one could rename the existing astropy package to e.g. astropy-core and then create a new astropy package which would be a metapackage that would contain the following dependencies section:
Since users may want to pin or place version constraints on the astropy meta-package (e.g. astropy>5.0), the metapackage would need to follow the same versions as the core package, and would need to use strict pinning in the dependency section, e.g.:
This idea may seem appealing because it is technically already feasible. However, in practice, many projects have opted not to do this, for a number of reasons, which we now take a look at. Some of these may not be applicable to future new projects, but some of them apply to all projects, old and new.
In terms of naming, there are two main options for a package that wants to use the metapackage approach:
but this would not be the case (the package module would still work), and it may not be obvious to this user that the package-core package even exists.
This approach requires either maintaining two repositories instead of one, or switching to using a monorepo which would contain both packages. Neither option is ideal:
Packages that need to depend on package versions that are older than the first version where the split was done will not easily be able to depend on the minimal package. Whereas with the main proposal in this PEP, downstream users will be able to depend on e.g. package[]>version where version pre-dates the introduction of default extras, with the splitting approach it will not be possible for downstream users to depend on e.g. package-core>version, since package-core did not previously exist.
A possible solution to this is for developers to release no-op metadata packages for all old versions of a package, but this is a significant additional burden on the maintainers.
As alluded to when referring to naming issues in Mismatch between package and module name, uninstalling packages will no longer work the way users expect. A user doing:
will still be left with package-core, but may not realise it. This is not just confusing, but is in effect a breaking change that may impact a number of existing workflows.
Having two packages instead of one would increase the long-term maintenance cost of package distributions simply by virtue of the fact that two packages would have to be released instead of one, and in some cases this would introduce extra manual work at each release.
The main metadata that would be important to keep synchronized between the main package and the metapackage is the version. Anytime a new release of the core package is done, the metapackage would need to have its version updated as well as the version pinning for the core package in the dependencies.
In addition, all extras defined in the core package would need to be redefined and kept in sync in the metapackage. For example, if package defines a additional extra, users should still be able to install package[additional], but users installing the package-core package should also have the option of doing package-core[additional].
Other metadata that would need to be kept in sync includes for example author information and project URLs.
Overall, this solution would imply a significantly higher maintenance burden, not just in terms of initial set-up and transition (which could already be prohibitive for large established projects), but also in terms of long-term maintenance. This also has the potential for breaking user workflows (in particular around the issue of repositories, and e.g. uninstallation). For all these reasons, we do not consider it a compelling alternative to the present PEP.
One of the main competing approaches was as follows: instead of having defaults be unselected if any extras were explicitly provided, default extras would need to be explicitly unselected.
In this picture, a new syntax for unselecting extras would be introduced as an extension of the mini-language defined in PEP 508. If a package defined default extras, users could opt out of these defaults by using a minus sign (-) before the extra name. The proposed syntax update would have been as follows:
Valid examples of this new syntax would have included, e.g.:
However, there are two main issues with this approach:
For these reasons, this alternative was not included in the final proposal.
A potential solution that has been explored as an alternative to introducing the new Default-Extra metadata field would be to make use of an extra with a ‘special’ name.
One example would be to use an empty string:
The idea would be that dependencies installed as part of the ‘empty’ extras would only get installed if another extra was not specified. An implementation of this was proposed in https://github.com/pypa/setuptools/pull/1503, but it was found that there would be no way to make this work without breaking compatibility with existing usage. For example, packages using Setuptools via a setup.py file can do:
which is valid and equivalent to having package_a being defined in install_requires, so changing the meaning of the empty string would break compatibility.
In addition, no other string (such as 'default') can be used as a special string since all strings that would be a backward-compatible valid extras name may already be used in existing packages.
There have been suggestions of using the special None Python variable, but again this is not possible, because even though one can use None in a setup.py file, this is not possible in declarative files such as setup.cfg or pyproject.toml, and furthermore ultimately extras names have to be converted to strings in the package metadata. Having:
would be indistinguishable from the string ‘None’ which may already be used as an extra name in a Python package. If we were to modify the core metadata syntax to allow non-string ‘special’ extras names, then we would be back to modifying the core metadata specification, which is no better than introducing Default-Extra.
Another option to unselect extras would be to implement this at the level of packaging tools. For instance, pip could include an option such as:
This option could apply to all or specific packages, similar to the --no-binary option, e.g.,:
The advantage of this approach is that tools supporting default extras could also support unselecting them. This approach would be similar to the --no-install-recommends option for the apt tool.
However, this solution is not ideal on its own because it would not allow packages to specify themselves that they do not need some of the default extras of a dependency. It would also carry risks for users who might disable all default extras in a big dependency tree, potentially breaking packages in the tree that rely on default extras at any point.
Nevertheless, this PEP does not disallow this approach and it is up to the maintainers of different packaging tools to decide if they want to support this kind of option. It is a flag that could at the very least be useful for package maintainers who want to identify places in dependency trees where default extras are being relied on. However, if it is supported, it should be made clear that using this flag does not guarantee a functional environment.
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-0771.rst
Last modified: 2025-06-09 11:29:19 UTC