PEP: 425 Title: Compatibility Tags for Built Distributions Version:
$Revision$ Last-Modified: 07-Aug-2012 Author: Daniel Holth
<dholth@gmail.com> BDFL-Delegate: Alyssa Coghlan <ncoghlan@gmail.com>
Status: Final Type: Standards Track Topic: Packaging Content-Type:
text/x-rst Created: 27-Jul-2012 Python-Version: 3.4 Post-History:
08-Aug-2012, 18-Oct-2012, 15-Feb-2013 Resolution:
https://mail.python.org/pipermail/python-dev/2013-February/124116.html

packaging:platform-compatibility-tags

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 Alyssa 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):

1.  cp33-cp33m-linux_x86_64
2.  cp33-abi3-linux_x86_64
3.  cp3-abi3-linux_x86_64
4.  cp33-none-linux_x86_64*
5.  cp3-none-linux_x86_64*
6.  py33-none-linux_x86_64*
7.  py3-none-linux_x86_64*
8.  cp33-none-any
9.  cp3-none-any
10. py33-none-any
11. py3-none-any
12. py32-none-any
13. py31-none-any
14. 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(du)" analogous to newer versions of Python, but
    in the meantime "none" is a good enough way to say "don't know".

References

[1] Egg Filename-Embedded Metadata
(http://peak.telecommunity.com/DevCenter/EggFormats#filename-embedded-metadata)

[2] Creating Built Distributions
(https://docs.python.org/3.4/distutils/builtdist.html)

Acknowledgements

The author thanks Paul Moore, Alyssa Coghlan, Marc Abramowitz, and Mr.
Michele Lacchia for their valuable help and advice.

Copyright

This document has been placed in the public domain.