PEP 396 – Module Version Numbers

PEP: 396
Title: Module Version Numbers
Author: Barry Warsaw <barry at python.org>
Status: Rejected
Type: Informational
Created: 16-Mar-2011
Post-History: 2011-04-05

Abstract
PEP Rejection
User Stories
Rationale
Specification
Examples
Deriving
- Classic distutils
- Distutils2
PEP 376 metadata
References
Copyright

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.

PEP Rejection

This PEP was formally rejected on 2021-04-14. The packaging ecosystem has changed significantly in the intervening years since this PEP was first written, and APIs such as importlib.metadata.versions() 11 provide for a much better experience.

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.

Carol 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 (or package) includes a version number, the version SHOULD be available in the __version__ attribute.
For modules which live inside a namespace package, the module SHOULD include the __version__ attribute. The namespace package 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.

In consultation with the distutils-sig 9, two options are proposed. Both entail containing the version number in a file, and declaring that file in the setup.cfg. When the entire contents of the file contains the version number, the version-file key will be used:

[metadata]
version-file: version.txt

When the version number is contained within a larger file, e.g. of Python code, such that the file must be parsed to extract the version, the key version-from-file will be used:

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

A parsing method similar to that described above will be performed 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 10 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.