Abstract

This PEP specifies a tagging system to indicate with which versions of Python a built or binary distribution is compatible. A set of three tags indicate which Python implementation and language version, ABI, and platform a built distribution requires. The tags are terse because they will be included in filenames.

PEP Acceptance

This PEP was accepted by Nick Coghlan on 17th February, 2013.

Rationale

Today “python setup.py bdist” generates the same filename on PyPy and CPython, but an incompatible archive, making it inconvenient to share built distributions in the same folder or index. Instead, built distributions should have a file naming convention that includes enough information to decide whether or not a particular archive is compatible with a particular implementation.

Previous efforts come from a time where CPython was the only important implementation and the ABI was the same as the Python language release. This specification improves upon the older schemes by including the Python implementation, language version, ABI, and platform as a set of tags.

By comparing the tags it supports with the tags listed by the distribution, an installer can make an educated decision about whether to download a particular built distribution without having to read its full metadata.

Overview

The tag format is {python tag}-{abi tag}-{platform tag}

python tag: ‘py27’, ‘cp33’
abi tag: ‘cp32dmu’, ‘none’
platform tag: ‘linux_x86_64’, ‘any’

For example, the tag py27-none-any indicates compatible with Python 2.7 (any Python 2.7 implementation) with no abi requirement, on any platform.

Use

The wheel built package format includes these tags in its filenames, of the form {distribution}-{version}(-{build tag})?-{python tag}-{abi tag}-{platform tag}.whl. Other package formats may have their own conventions.

Details

Python Tag

The Python tag indicates the implementation and version required by a distribution. Major implementations have abbreviated codes, initially:

py: Generic Python (does not require implementation-specific features)
cp: CPython
ip: IronPython
pp: PyPy
jy: Jython

Other Python implementations should use sys.implementation.name.

The version is py_version_nodot. CPython gets away with no dot, but if one is needed the underscore _ is used instead. PyPy should probably use its own versions here pp18, pp19.

The version can be just the major version 2 or 3 py2, py3 for many pure-Python distributions.

Importantly, major-version-only tags like py2 and py3 are not shorthand for py20 and py30. Instead, these tags mean the packager intentionally released a cross-version-compatible distribution.

A single-source Python 2/3 compatible distribution can use the compound tag py2.py3. See Compressed Tag Sets, below.

ABI Tag

The ABI tag indicates which Python ABI is required by any included extension modules. For implementation-specific ABIs, the implementation is abbreviated in the same way as the Python Tag, e.g. cp33d would be the CPython 3.3 ABI with debugging.

The CPython stable ABI is abi3 as in the shared library suffix.

Implementations with a very unstable ABI may use the first 6 bytes (as 8 base64-encoded characters) of the SHA-256 hash of their source code revision and compiler flags, etc, but will probably not have a great need to distribute binary distributions. Each implementation’s community may decide how to best use the ABI tag.

Platform Tag

The platform tag is simply distutils.util.get_platform() with all hyphens - and periods . replaced with underscore _.

win32
linux_i386
linux_x86_64

Use

The tags are used by installers to decide which built distribution (if any) to download from a list of potential built distributions. The installer maintains a list of (pyver, abi, arch) tuples that it will support. If the built distribution’s tag is in the list, then it can be installed.

It is recommended that installers try to choose the most feature complete built distribution available (the one most specific to the installation environment) by default before falling back to pure Python versions published for older Python releases. Installers are also recommended to provide a way to configure and re-order the list of allowed compatibility tags; for example, a user might accept only the *-none-any tags to only download built packages that advertise themselves as being pure Python.

Another desirable installer feature might be to include “re-compile from source if possible” as more preferable than some of the compatible but legacy pre-built options.

This example list is for an installer running under CPython 3.3 on a linux_x86_64 system. It is in order from most-preferred (a distribution with a compiled extension module, built for the current version of Python) to least-preferred (a pure-Python distribution built with an older version of Python):

cp33-cp33m-linux_x86_64
cp33-abi3-linux_x86_64
cp3-abi3-linux_x86_64
cp33-none-linux_x86_64*
cp3-none-linux_x86_64*
py33-none-linux_x86_64*
py3-none-linux_x86_64*
cp33-none-any
cp3-none-any
py33-none-any
py3-none-any
py32-none-any
py31-none-any
py30-none-any

Built distributions may be platform specific for reasons other than C extensions, such as by including a native executable invoked as a subprocess.

Sometimes there will be more than one supported built distribution for a particular version of a package. For example, a packager could release a package tagged cp33-abi3-linux_x86_64 that contains an optional C extension and the same distribution tagged py3-none-any that does not. The index of the tag in the supported tags list breaks the tie, and the package with the C extension is installed in preference to the package without because that tag appears first in the list.

Compressed Tag Sets

To allow for compact filenames of bdists that work with more than one compatibility tag triple, each tag in a filename can instead be a ‘.’-separated, sorted, set of tags. For example, pip, a pure-Python package that is written to run under Python 2 and 3 with the same source code, could distribute a bdist with the tag py2.py3-none-any. The full list of simple tags is:

for x in pytag.split('.'):
    for y in abitag.split('.'):
        for z in archtag.split('.'):
            yield '-'.join((x, y, z))

A bdist format that implements this scheme should include the expanded tags in bdist-specific metadata. This compression scheme can generate large numbers of unsupported tags and “impossible” tags that are supported by no Python implementation e.g. “cp33-cp31u-win64”, so use it sparingly.

FAQ

What tags are used by default?: Tools should use the most-preferred architecture dependent tag e.g. cp33-cp33m-win32 or the most-preferred pure python tag e.g. py33-none-any by default. If the packager overrides the default it indicates that they intended to provide cross-Python compatibility.
What tag do I use if my distribution uses a feature exclusive to the newest version of Python?: Compatibility tags aid installers in selecting the most compatible build of a single version of a distribution. For example, when there is no Python 3.3 compatible build of beaglevote-1.2.0 (it uses a Python 3.4 exclusive feature) it may still use the py3-none-any tag instead of the py34-none-any tag. A Python 3.3 user must combine other qualifiers, such as a requirement for the older release beaglevote-1.1.0 that does not use the new feature, to get a compatible build.
Why isn’t there a . in the Python version number?: CPython has lasted 20+ years without a 3-digit major release. This should continue for some time. Other implementations may use _ as a delimiter, since both - and . delimit the surrounding filename.
Why normalise hyphens and other non-alphanumeric characters to underscores?: To avoid conflicting with the “.” and “-” characters that separate components of the filename, and for better compatibility with the widest range of filesystem limitations for filenames (including being usable in URL paths without quoting).
Why not use special character <X> rather than “.” or “-“?: Either because that character is inconvenient or potentially confusing in some contexts (for example, “+” must be quoted in URLs, “~” is used to denote the user’s home directory in POSIX), or because the advantages weren’t sufficiently compelling to justify changing the existing reference implementation for the wheel format defined in PEP 427 (for example, using “,” rather than “.” to separate components in a compressed tag).
Who will maintain the registry of abbreviated implementations?: New two-letter abbreviations can be requested on the python-dev mailing list. As a rule of thumb, abbreviations are reserved for the current 4 most prominent implementations.
Does the compatibility tag go into METADATA or PKG-INFO?: No. The compatibility tag is part of the built distribution’s metadata. METADATA / PKG-INFO should be valid for an entire distribution, not a single build of that distribution.
Why didn’t you mention my favorite Python implementation?: The abbreviated tags facilitate sharing compiled Python code in a public index. Your Python implementation can use this specification too, but with longer tags. Recall that all “pure Python” built distributions just use ‘py’.
Why is the ABI tag (the second tag) sometimes “none” in the reference implementation?: Since Python 2 does not have an easy way to get to the SOABI (the concept comes from newer versions of Python 3) the reference implementation at the time of writing guesses “none”. Ideally it would detect “py27(d|m|u)” analogous to newer versions of Python, but in the meantime “none” is a good enough way to say “don’t know”.