python-peps/pep-0396.txt

PEP: 396
Title: Module Version Numbers
Version: $Revision: 65628 $
Last-Modified: $Date: 2008-08-10 09:59:20 -0400 (Sun, 10 Aug 2008) $
Author: Barry Warsaw <barry@python.org>
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 2011-03-16
Post-History: 2011-04-05


Abstract
========

Given that it is useful and common to specify version numbers for
Python modules, and given that different ways of doing this have grown
organically within the Python community, it is useful to establish
standard conventions for module authors to adhere to and reference.
This informational PEP describes best practices for Python module
authors who want to define the version number of their Python module.

Conformance with this PEP is optional, however other Python tools
(such as ``distutils2`` [1]_) may be adapted to use the conventions
defined here.


User Stories
============

Alice is writing a new module, called ``alice``, which she wants to
share with other Python developers.  ``alice`` is a simple module and
lives in one file, ``alice.py``.  Alice wants to specify a version
number so that her users can tell which version they are using.
Because her module lives entirely in one file, she wants to add the
version number to that file.

Bob has written a module called ``bob`` which he has shared with many
users.  ``bob.py`` contains a version number for the convenience of
his users.  Bob learns about the Cheeseshop [2]_, and adds some simple
packaging using classic distutils so that he can upload *The Bob
Bundle* to the Cheeseshop.  Because ``bob.py`` already specifies a
version number which his users can access programmatically, he wants
the same API to continue to work even though his users now get it from
the Cheeseshop.

Carole maintains several namespace packages, each of which are
independently developed and distributed.  In order for her users to
properly specify dependencies on the right versions of her packages,
she specifies the version numbers in the namespace package's
``setup.py`` file.  Because Carol wants to have to update one version
number per package, she specifies the version number in her module and
has the ``setup.py`` extract the module version number when she builds
the *sdist* archive.

David maintains a package in the standard library, and also produces
standalone versions for other versions of Python.  The standard
library copy defines the version number in the module, and this same
version number is used for the standalone distributions as well.


Rationale
=========

Python modules, both in the standard library and available from third
parties, have long included version numbers.  There are established
de-facto standards for describing version numbers, and many ad-hoc
ways have grown organically over the years.  Often, version numbers
can be retrieved from a module programmatically, by importing the
module and inspecting an attribute.  Classic Python distutils
``setup()`` functions [3]_ describe a ``version`` argument where the
release's version number can be specified.  PEP 8 [4]_ describes the
use of a module attribute called ``__version__`` for recording
"Subversion, CVS, or RCS" version strings using keyword expansion.  In
the PEP author's own email archives, the earliest example of the use
of an ``__version__`` module attribute by independent module
developers dates back to 1995.

Another example of version information is the sqlite3 [5]_ module
with its ``sqlite_version_info``, ``version``, and ``version_info``
attributes.  It may not be immediately obvious which attribute
contains a version number for the module, and which contains a version
number for the underlying SQLite3 library.

This informational PEP codifies established practice, and recommends
standard ways of describing module version numbers, along with some
use cases for when -- and when *not* -- to include them.  Its adoption
by module authors is purely voluntary; packaging tools in the standard
library will provide optional support for the standards defined
herein, and other tools in the Python universe may comply as well.


Specification
=============

#. In general, modules in the standard library SHOULD NOT have version
   numbers.  They implicitly carry the version number of the Python
   release they are included in.

#. On a case-by-case basis, standard library modules which are also
   released in standalone form for other Python versions MAY include a
   module version number when included in the standard library, and
   SHOULD include a version number when packaged separately.

#. When a module includes a version number, it SHOULD be available in
   the ``__version__`` attribute on that module.

#. For modules which are also packages, the module namespace SHOULD
   include the ``__version__`` attribute.

#. For modules which live inside a namespace package, the sub-package
   name SHOULD include the ``__version__`` attribute.  The namespace
   module itself SHOULD NOT include its own ``__version__`` attribute.

#. The ``__version__`` attribute's value SHOULD be a string.

#. Module version numbers SHOULD conform to the normalized version
   format specified in PEP 386 [6]_.

#. Module version numbers SHOULD NOT contain version control system
   supplied revision numbers, or any other semantically different
   version numbers (e.g. underlying library version number).

#. The ``version`` attribute in a classic distutils ``setup.py``
   file, or the PEP 345 [7]_ ``Version`` metadata field SHOULD be
   derived from the ``__version__`` field, or vice versa.


Examples
========

Retrieving the version number from a third party package::

    >>> import bzrlib
    >>> bzrlib.__version__
    '2.3.0'

Retrieving the version number from a standard library package that is
also distributed as a standalone module::

    >>> import email
    >>> email.__version__
    '5.1.0'

Version numbers for namespace packages::

    >>> import flufl.i18n
    >>> import flufl.enum
    >>> import flufl.lock

    >>> print flufl.i18n.__version__
    1.0.4
    >>> print flufl.enum.__version__
    3.1
    >>> print flufl.lock.__version__
    2.1

    >>> import flufl
    >>> flufl.__version__
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'module' object has no attribute '__version__'
    >>>


Deriving
========

Module version numbers can appear in at least two places, and
sometimes more.  For example, in accordance with this PEP, they are
available programmatically on the module's ``__version__`` attribute.
In a classic distutils ``setup.py`` file, the ``setup()`` function
takes a ``version`` argument, while the distutils2 ``setup.cfg`` file
has a ``version`` key.  The version number must also get into the PEP
345 metadata, preferably when the *sdist* archive is built.  It's
desirable for module authors to only have to specify the version
number once, and have all the other uses derive from this single
definition.

This could be done in any number of ways, a few of which are outlined
below.  These are included for illustrative purposes only and are not
intended to be definitive, complete, or all-encompassing.  Other
approaches are possible, and some included below may have limitations
that prevent their use in some situations.

Let's say Elle adds this attribute to her module file ``elle.py``::

    __version__ = '3.1.1'


Classic distutils
-----------------

In classic distutils, the simplest way to add the version string to
the ``setup()`` function in ``setup.py`` is to do something like
this::

    from elle import __version__
    setup(name='elle', version=__version__)

In the PEP author's experience however, this can fail in some cases,
such as when the module uses automatic Python 3 conversion via the
``2to3`` program (because ``setup.py`` is executed by Python 3 before
the ``elle`` module has been converted).

In that case, it's not much more difficult to write a little code to
parse the ``__version__`` from the file rather than importing it.
Without providing too much detail, it's likely that modules such as
``distutils2`` will provide a way to parse version strings from files.
E.g.::

    from distutils2 import get_version
    setup(name='elle', version=get_version('elle.py'))


Distutils2
----------

Because the distutils2 style ``setup.cfg`` is declarative, we can't
run any code to extract the ``__version__`` attribute, either via
import or via parsing.  This PEP suggests a special key be added to
the ``[metadata]`` section of the ``setup.cfg`` file to indicate "get
the version from this file".  Something like this might work::

    [metadata]
    version-from-file: elle.py

where ``parse`` means to use a parsing method similar to the above, on
the file named after the colon.  The exact recipe for doing this will
be discussed in the appropriate distutils2 development forum.

An alternative is to only define the version number in ``setup.cfg``
and use the ``pkgutil`` module [8]_ to make it available
programmatically.  E.g. in ``elle.py``::

    from distutils2._backport import pkgutil
    __version__ = pkgutil.get_distribution('elle').metadata['version']


PEP 376 metadata
================

PEP 376 [9]_ defines a standard for static metadata, but doesn't
describe the process by which this metadata gets created.  It is
highly desirable for the derived version information to be placed into
the PEP 376 ``.dist-info`` metadata at build-time rather than
install-time.  This way, the metadata will be available for
introspection even when the code is not installed.


References
==========

.. [1] Distutils2 documentation
   (http://distutils2.notmyidea.org/)

.. [2] The Cheeseshop (Python Package Index)
   (http://pypi.python.org)

.. [3] http://docs.python.org/distutils/setupscript.html

.. [4] PEP 8, Style Guide for Python Code
   (http://www.python.org/dev/peps/pep-0008)

.. [5] sqlite3 module documentation
   (http://docs.python.org/library/sqlite3.html)

.. [6] PEP 386, Changing the version comparison module in Distutils
   (http://www.python.org/dev/peps/pep-0386/)

.. [7] PEP 345, Metadata for Python Software Packages 1.2
   (http://www.python.org/dev/peps/pep-0345/#version)

.. [8] pkgutil - Package utilities
   (http://distutils2.notmyidea.org/library/pkgutil.html)

.. [9] PEP 376, Database of Installed Python Distributions
   (http://www.python.org/dev/peps/pep-0376/)


Copyright
=========

This document has been placed in the public domain.



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