Add Wheel-Version Metadata Field to Core Metadata

Currently, the wheel 1.0 PEP, PEP 427, specifies that wheel files must contain a WHEEL metadata file that contains the version of the wheel specification that the file conforms to. PEP 427 stipulates that installers MUST warn on installation of a wheel with a minor version greater than supported, and MUST abort on installation of wheels with a major version that is greater than what the installer supports. This ensures that users do not get invalid installations from wheels that installers cannot properly install.

However, resolvers do not currently exclude wheels with an incompatible wheel version. There is also currently no way for a resolver to check a wheel’s version without downloading the wheel directly. To make wheel version filtering easy for resolvers, the wheel version MUST be included in the relevant metadata file (currently METADATA). This will allow resolvers to efficiently check the wheel version using the PEP 658 metadata API without needing to download and inspect the .dist-info/WHEEL file.

To accomplish this, a new field, Wheel-Version, will be added to the Core Metadata Specification. This field is single use, and must contain the exact same version specified as the Wheel-Version entry in the WHEEL file, or any future replacement file defining metadata about the wheel file. If Wheel-Version is absent from the metadata file, then tools MUST infer the wheel file major version as 1.

Wheel-Version MUST NOT be included in source distribution metadata (PKG-INFO) files. If a tool encounters Wheel-Version inside of a source distribution metadata file, it SHOULD raise an error.

Wheel-Version MAY be included in the metadata file for wheels of version 1, but for wheels of version 2 or higher, the metadata file MUST include Wheel-Version. This enforces that future revisions of the wheel specification can rely on resolvers skipping incompatible wheels by checking the Wheel-Version field. Build backends are encouraged to include Wheel-Version in all wheels that they generate, regardless of version.

Installers SHOULD copy the metadata file in a wheel unmodified during installation. This prevents the need to update the RECORD file, which is an error prone process. Tools reading installed core metadata SHOULD NOT assume that the field is present, as other installation formats may omit it.

When installing a wheel, installers MUST take the following steps:

1. Check that the values of Wheel-Version in both the core metadata file and wheel metadata file match. If they do not match, installers MUST abort installation. Neither value takes precedence.
2. Check that the installer is compatible with Wheel-Version. If Wheel-Version is absent, assume the version is 1.0. Warn if minor version is greater, abort if major version is greater. This procedure is identital to that in PEP 427.
3. Proceed with installation as specified in the Binary Distribution Format specification.

Resolver Behavior Regarding Wheel-Version

Resolvers, in the process of selecting a wheel to install, MUST check a candidate wheel’s Wheel-Version, and ignore incompatible wheel files. Without ignoring these files, older installers might select a wheel that uses an unsupported wheel version for that installer, and force the installer to abort per PEP 427. By skipping incompatible wheel files, users will not see installation errors when a project adopts a new wheel major version. As already specified in PEP 427, installers MUST abort if a user tries to directly install a wheel that is incompatible. If, in the process of resolving packages found in multiple indices, a resolver comes across two wheels of the same distribution and version, resolvers should prioritize the wheel of the highest compatible version.

While the above protects users from unexpected breakages, users may miss a new release of a distribution if their installer does not support the wheel version used in the release. Imagine in the future that a package publishes 3.0 wheel files. Downstream users won’t see that there is a new release available if their installers only support 2.x wheels. Therefore, installers SHOULD emit a warning if, in the process of resolving packages, they come across an incompatible wheel and skip it.

First Major Version Bump Must Change File Extension

Unfortunately, existing resolvers do not check the compatibility of wheels before selecting them as installation candidates. Until a majority of users update to installers that properly check for wheel compatibility, it is unsafe to allow publishing wheels of a new major version that existing resolvers might select. It could take upwards of four years before the majority of users are on updated resolvers, based on current data about PyPI installer usage (See the Appendix: Analysis of Installer Usage on PyPI, for details). To allow for experimentation and faster adoption of 2.0 wheels, this PEP proposes a change to the file extension of the wheel file format, from .whl to .whlx for all future wheel versions. Note that x in whlx is the letter “x” and does not specify the wheel major version. The change to extension name resolves the initial transition issue of 2.0 wheels breaking users on existing installers that do not implement Wheel-Version checks. By using a different file extension, 2.0 wheels can immediately be uploaded to PyPI, and users will be able to experiment with the new features right away. Users on older installers will simply ignore these new files.

One rejected alternative would be to keep the .whl extension, but delay the publishing of wheel 2.0 to PyPI. For more on that, please see Rejected Ideas.

Recommended Build Backend Behavior with New Wheel Formats

Build backends are recommended to generate the most compatible wheel based on features a project uses. For example, if a wheel does not use symbolic links, and such a feature was introduced in wheel 5.0, the build backend could generate a wheel of version 4.0. On the other hand, some features will want to be adopted by default. For example, if wheel 3.0 introduces better compression, the build backend may wish to enable this feature by default to improve the wheel size and download performance.

Limitations on Future Wheel Revisions

While it is difficult to know what future features may be planned for the wheel format, it is important that certain compatibility promises are maintained.

Wheel files, when installed, MUST stay compatible with the Python standard library’s importlib.metadata for all supported CPython versions. For example, replacing .dist-info/METADATA with a JSON formatted metadata file MUST be a multi-major version migration with one version introducing the new JSON file alongside the existing email header format, and another future version removing the email header format metadata file. The version to remove .dist-info/METADATA also MUST be adopted only after the last CPython release that lacked support for the new file reaches end of life. This ensures that code using importlib.metadata will not break with wheel major version revisions.

Wheel files MUST remain ZIP format files as the outer container format. Additionally, the .dist-info metadata directory MUST be placed at the root of the archive without any compression, so that unpacking the wheel file produces a normal .dist-info directory holding any metadata for the wheel. Future wheel revisions MAY modify the layout, compression, and other attributes about non-metadata components of a wheel such as data and code. This assures that future wheel revisions remain compatible with tools operating on package metadata, while allowing for improvements to code storage in the wheel, such as adopting compression.

Package tooling MUST NOT assume that the contents and format of the wheel file will remain the same for future wheel major versions beyond the limitations above about metadata folder contents and outer container format. For example, newer wheel major versions may add or remove filename components, such as the build tag or the platform tag. Therefore it is incumbent upon tooling to check the metadata for the Wheel-Version before attempting to install a wheel.

Finally, future wheel revisions MUST NOT use any compression formats not in the CPython standard library of at least the latest release. Wheels generated using any new compression format should be tagged as requiring at least the first released version of CPython to support the new compression format, regardless of the Python API compatibility of the code within the wheel.

The Wheel Format is Perfect and Does not Need to be Changed

The wheel format has been around for over 10 years, and in that time, Python packages have changed a lot. It is much more common for packages to include Rust or C extension modules, increasing the size of packages. Better compression, such as lzma or zstd, could save a lot of time and bandwidth for PyPI and its users. Compatibility tags cannot express the wide variety of hardware used to accelerate Python code today, nor encode shared library compatibility information. In order to address these issues, evolution of the wheel package format is necessary.

Wheel Format Changes Should be Tied to CPython Releases

I do not believe that tying wheel revisions to CPython releases is beneficial. The main benefit of doing so is to make adoption of new wheels predictable - users with the latest CPython get the latest package format! This choice has several issues however. First, tying the new format to the latest CPython makes adoption much slower. Users on LTS versions of Linux with older Python installations are free to update their pip in a virtual environment, but cannot update the version of Python as easily. While some changes to the wheel format must be tied to CPython changes necessarily, such as adding new compression formats or changing the metadata format, many changes do not need to be tied to the Python version, such as symlinks, enhanced compatibility tags, and new formats that use existing compression formats in the standard library. Additionally, wheels are used across multiple different language implementations, which lag behind the CPython version. It seems unfair to prevent their users from using a feature due to the Python version. Lastly, while this PEP does not suggest tying the wheel version to CPython releases, a future PEP may still do so at any time, so this choice does not need to be made in this PEP.

Keep Using .whl as the File Extension

While keeping the extension .whl is appealing for many reasons, it presents several problems that are difficult to surmount. First, current installers would still pick a new wheel and fail to install the package. Furthermore, the file name of a wheel would not be able to change without breaking existing installers that expect a set wheel file name format. While the current filename specification for wheels is sufficient for current usage, the optional build tag in the middle of the file name makes any extensions ambiguous (i.e. foo-0.3-py3-none-any-fancy_new_tag.whl would parse as the build tag being py3). This limits changes to information stored in the wheel file name.

Store the Wheel Major Version in the File Extension (.whl2)

Storing the wheel major version in the file extension has several nice advantages. For one, there is no need to introduce the Wheel-Version metadata field, since installers could simply filter based on file extension. This would also allow future side-by-side packages. However, changing the extension for wheels each major version has some downsides. First, the version stored in the WHEEL file must match the file extension, and this would need to be verified by installers. Additionally, many systems associate file type by file extension (e.g. executable associations, various web caching software), and these would need to be updated every version that is released. Furthermore, part of the brittleness of the current wheel specification is that so much metadata is stored in the filename. Filenames are not well suited to store structured data. Moving away from encoding information in the filename should be a goal of future wheel revisions.

Another possibility is to use the file extension to encode the outer container format (i.e. a ZIP file containing .dist-info) separate from the inner wheel version. However, this could lead to confusion if the file extension and inner Wheel-Version diverge. If an installer raises an error due to an incompatible wheel 3.0 as obtained from the wheel metadata, some users will be confused by the difference from the file extension .whl2.

Wheel 2.0 Should Change the Outer Container Format

Since wheel 2.0 will change the extension of wheel files, it is the best opportunity to modify the outer container format. Compatibility does not need to be kept with a different file extension that tools will need to opt-in to reading. The main use-case for a different exterior compression format would be better compression. For example, the outer container could be changed into a Zstandard tarfile, .tar.zst, which would decompress faster and produce smaller wheels. However, there are several practical issues with this. First, Zstandard is not part of the Python standard library, so pure-Python packaging tools would need to ship an extension to unpack these wheels. This could cause some compatibility issues for several platforms where extension modules are not easy to install. Furthermore, a future wheel revision could always introduce a new layout of non-metadata files that uses a .tar.zst inside the existing ZIP-based format.

Finally, it is not a good idea to change the wheel file format too much at once. The goal of this PEP is to make evolving the specification easier, and part of the rationale behind making wheel evolution easier is to avoid “all at once” changes. Changing the outer file format for wheels would require re-writing how package metadata is not only discovered, but also installed.

Why not Specify Wheel 2.0 In This PEP?

There are many features that could be included as part of wheel 2.0, but this PEP does not cover them. The goal of this PEP is to define a compatibility story for the wheel file format. Changes that do not pertain to compatibility for wheel versions do not need to be in this PEP, and should be introducted in follow-up PEPs defining new wheel features.

Should Indices Support Dual-publishing for the First Migration?

Since .whl and .whlx will look different in file name, they could be uploaded side-by-side to package indices like PyPI. This has some nice benefits, like dual-support for older and newer installers, so users who can get the latest features, while users who don’t upgrade still can install the latest version of a package.

There are many complications however. Should we allow wheel 2 uploads to existing wheel 1-only releases? Should we put any requirements on the side-by-side wheels, such as:

Should we only allow uploading both with PEP 694 allowing “atomic” dual-publishing?