293 lines
11 KiB
Plaintext
293 lines
11 KiB
Plaintext
PEP: 425
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Title: Compatibility Tags for Built Distributions
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Version: $Revision$
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Last-Modified: 07-Aug-2012
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Author: Daniel Holth <dholth@gmail.com>
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BDFL-Delegate: Nick Coghlan <ncoghlan@gmail.com>
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Status: Final
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Type: Standards Track
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Topic: Packaging
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Content-Type: text/x-rst
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Created: 27-Jul-2012
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Python-Version: 3.4
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Post-History: 08-Aug-2012, 18-Oct-2012, 15-Feb-2013
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Resolution: https://mail.python.org/pipermail/python-dev/2013-February/124116.html
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.. canonical-pypa-spec:: :ref:`packaging:platform-compatibility-tags`
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Abstract
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========
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This PEP specifies a tagging system to indicate with which versions of
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Python a built or binary distribution is compatible. A set of three
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tags indicate which Python implementation and language version, ABI,
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and platform a built distribution requires. The tags are terse because
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they will be included in filenames.
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PEP Acceptance
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==============
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This PEP was accepted by Nick Coghlan on 17th February, 2013.
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Rationale
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=========
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Today "python setup.py bdist" generates the same filename on PyPy
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and CPython, but an incompatible archive, making it inconvenient to
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share built distributions in the same folder or index. Instead, built
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distributions should have a file naming convention that includes enough
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information to decide whether or not a particular archive is compatible
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with a particular implementation.
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Previous efforts come from a time where CPython was the only important
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implementation and the ABI was the same as the Python language release.
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This specification improves upon the older schemes by including the Python
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implementation, language version, ABI, and platform as a set of tags.
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By comparing the tags it supports with the tags listed by the
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distribution, an installer can make an educated decision about whether
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to download a particular built distribution without having to read its
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full metadata.
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Overview
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========
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The tag format is {python tag}-{abi tag}-{platform tag}
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python tag
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‘py27’, ‘cp33’
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abi tag
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‘cp32dmu’, ‘none’
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platform tag
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‘linux_x86_64’, ‘any’
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For example, the tag py27-none-any indicates compatible with Python 2.7
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(any Python 2.7 implementation) with no abi requirement, on any platform.
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Use
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===
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The ``wheel`` built package format includes these tags in its filenames,
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of the form ``{distribution}-{version}(-{build tag})?-{python tag}-{abi
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tag}-{platform tag}.whl``. Other package formats may have their own
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conventions.
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Details
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=======
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Python Tag
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----------
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The Python tag indicates the implementation and version required by
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a distribution. Major implementations have abbreviated codes, initially:
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* py: Generic Python (does not require implementation-specific features)
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* cp: CPython
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* ip: IronPython
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* pp: PyPy
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* jy: Jython
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Other Python implementations should use ``sys.implementation.name``.
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The version is ``py_version_nodot``. CPython gets away with no dot,
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but if one is needed the underscore ``_`` is used instead. PyPy should
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probably use its own versions here ``pp18``, ``pp19``.
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The version can be just the major version ``2`` or ``3`` ``py2``, ``py3`` for
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many pure-Python distributions.
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Importantly, major-version-only tags like ``py2`` and ``py3`` are not
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shorthand for ``py20`` and ``py30``. Instead, these tags mean the packager
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intentionally released a cross-version-compatible distribution.
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A single-source Python 2/3 compatible distribution can use the compound
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tag ``py2.py3``. See ``Compressed Tag Sets``, below.
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ABI Tag
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-------
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The ABI tag indicates which Python ABI is required by any included
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extension modules. For implementation-specific ABIs, the implementation
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is abbreviated in the same way as the Python Tag, e.g. ``cp33d`` would be
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the CPython 3.3 ABI with debugging.
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The CPython stable ABI is ``abi3`` as in the shared library suffix.
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Implementations with a very unstable ABI may use the first 6 bytes (as
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8 base64-encoded characters) of the SHA-256 hash of their source code
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revision and compiler flags, etc, but will probably not have a great need
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to distribute binary distributions. Each implementation's community may
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decide how to best use the ABI tag.
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Platform Tag
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------------
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The platform tag is simply ``distutils.util.get_platform()`` with all
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hyphens ``-`` and periods ``.`` replaced with underscore ``_``.
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* win32
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* linux_i386
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* linux_x86_64
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Use
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===
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The tags are used by installers to decide which built distribution
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(if any) to download from a list of potential built distributions.
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The installer maintains a list of (pyver, abi, arch) tuples that it
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will support. If the built distribution's tag is ``in`` the list, then
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it can be installed.
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It is recommended that installers try to choose the most feature complete
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built distribution available (the one most specific to the installation
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environment) by default before falling back to pure Python versions
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published for older Python releases. Installers are also recommended to
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provide a way to configure and re-order the list of allowed compatibility
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tags; for example, a user might accept only the ``*-none-any`` tags to only
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download built packages that advertise themselves as being pure Python.
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Another desirable installer feature might be to include "re-compile from
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source if possible" as more preferable than some of the compatible but
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legacy pre-built options.
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This example list is for an installer running under CPython 3.3 on a
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linux_x86_64 system. It is in order from most-preferred (a distribution
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with a compiled extension module, built for the current version of
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Python) to least-preferred (a pure-Python distribution built with an
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older version of Python):
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1. cp33-cp33m-linux_x86_64
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2. cp33-abi3-linux_x86_64
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3. cp3-abi3-linux_x86_64
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4. cp33-none-linux_x86_64*
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5. cp3-none-linux_x86_64*
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6. py33-none-linux_x86_64*
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7. py3-none-linux_x86_64*
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8. cp33-none-any
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9. cp3-none-any
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10. py33-none-any
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11. py3-none-any
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12. py32-none-any
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13. py31-none-any
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14. py30-none-any
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* Built distributions may be platform specific for reasons other than C
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extensions, such as by including a native executable invoked as
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a subprocess.
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Sometimes there will be more than one supported built distribution for a
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particular version of a package. For example, a packager could release
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a package tagged ``cp33-abi3-linux_x86_64`` that contains an optional C
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extension and the same distribution tagged ``py3-none-any`` that does not.
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The index of the tag in the supported tags list breaks the tie, and the
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package with the C extension is installed in preference to the package
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without because that tag appears first in the list.
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Compressed Tag Sets
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===================
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To allow for compact filenames of bdists that work with more than
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one compatibility tag triple, each tag in a filename can instead be a
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'.'-separated, sorted, set of tags. For example, pip, a pure-Python
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package that is written to run under Python 2 and 3 with the same source
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code, could distribute a bdist with the tag ``py2.py3-none-any``.
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The full list of simple tags is::
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for x in pytag.split('.'):
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for y in abitag.split('.'):
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for z in archtag.split('.'):
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yield '-'.join((x, y, z))
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A bdist format that implements this scheme should include the expanded
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tags in bdist-specific metadata. This compression scheme can generate
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large numbers of unsupported tags and "impossible" tags that are supported
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by no Python implementation e.g. "cp33-cp31u-win64", so use it sparingly.
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FAQ
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===
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What tags are used by default?
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Tools should use the most-preferred architecture dependent tag
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e.g. ``cp33-cp33m-win32`` or the most-preferred pure python tag
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e.g. ``py33-none-any`` by default. If the packager overrides the
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default it indicates that they intended to provide cross-Python
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compatibility.
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What tag do I use if my distribution uses a feature exclusive to the newest version of Python?
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Compatibility tags aid installers in selecting the *most compatible*
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build of a *single version* of a distribution. For example, when
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there is no Python 3.3 compatible build of ``beaglevote-1.2.0``
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(it uses a Python 3.4 exclusive feature) it may still use the
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``py3-none-any`` tag instead of the ``py34-none-any`` tag. A Python
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3.3 user must combine other qualifiers, such as a requirement for the
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older release ``beaglevote-1.1.0`` that does not use the new feature,
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to get a compatible build.
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Why isn't there a ``.`` in the Python version number?
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CPython has lasted 20+ years without a 3-digit major release. This
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should continue for some time. Other implementations may use _ as
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a delimiter, since both - and . delimit the surrounding filename.
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Why normalise hyphens and other non-alphanumeric characters to underscores?
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To avoid conflicting with the "." and "-" characters that separate
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components of the filename, and for better compatibility with the
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widest range of filesystem limitations for filenames (including
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being usable in URL paths without quoting).
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Why not use special character <X> rather than "." or "-"?
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Either because that character is inconvenient or potentially confusing
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in some contexts (for example, "+" must be quoted in URLs, "~" is
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used to denote the user's home directory in POSIX), or because the
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advantages weren't sufficiently compelling to justify changing the
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existing reference implementation for the wheel format defined in PEP
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427 (for example, using "," rather than "." to separate components
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in a compressed tag).
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Who will maintain the registry of abbreviated implementations?
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New two-letter abbreviations can be requested on the python-dev
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mailing list. As a rule of thumb, abbreviations are reserved for
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the current 4 most prominent implementations.
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Does the compatibility tag go into METADATA or PKG-INFO?
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No. The compatibility tag is part of the built distribution's
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metadata. METADATA / PKG-INFO should be valid for an entire
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distribution, not a single build of that distribution.
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Why didn't you mention my favorite Python implementation?
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The abbreviated tags facilitate sharing compiled Python code in a
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public index. Your Python implementation can use this specification
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too, but with longer tags.
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Recall that all "pure Python" built distributions just use 'py'.
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Why is the ABI tag (the second tag) sometimes "none" in the reference implementation?
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Since Python 2 does not have an easy way to get to the SOABI
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(the concept comes from newer versions of Python 3) the reference
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implementation at the time of writing guesses "none". Ideally it
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would detect "py27(d|m|u)" analogous to newer versions of Python,
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but in the meantime "none" is a good enough way to say "don't know".
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References
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==========
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[1] Egg Filename-Embedded Metadata
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\ (http://peak.telecommunity.com/DevCenter/EggFormats#filename-embedded-metadata)
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[2] Creating Built Distributions
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\ (https://docs.python.org/3.4/distutils/builtdist.html)
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Acknowledgements
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================
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The author thanks Paul Moore, Nick Coghlan, Marc Abramowitz, and
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Mr. Michele Lacchia for their valuable help and advice.
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Copyright
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=========
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This document has been placed in the public domain.
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