.. _extending-distutils:

*******************

Extending Distutils

*******************

Distutils can be extended in various ways.  Most extensions take the form of new

commands or replacements for existing commands.  New commands may be written to

support new types of platform-specific packaging, for example, while

replacements for existing commands may be made to modify details of how the

command operates on a package.

Most extensions of the distutils are made within :file:`setup.py` scripts that

want to modify existing commands; many simply add a few file extensions that

should be copied into packages in addition to :file:`.py` files as a

convenience.

Most distutils command implementations are subclasses of the :class:`Command`

class from :mod:`distutils.cmd`.  New commands may directly inherit from

:class:`Command`, while replacements often derive from :class:`Command`

indirectly, directly subclassing the command they are replacing.  Commands are

required to derive from :class:`Command`.

.. % \section{Extending existing commands}

.. % \label{extend-existing}

.. % \section{Writing new commands}

.. % \label{new-commands}

.. % \XXX{Would an uninstall command be a good example here?}

Integrating new commands

========================

There are different ways to integrate new command implementations into

distutils.  The most difficult is to lobby for the inclusion of the new features

in distutils itself, and wait for (and require) a version of Python that

provides that support.  This is really hard for many reasons.

The most common, and possibly the most reasonable for most needs, is to include

the new implementations with your :file:`setup.py` script, and cause the

:func:`distutils.core.setup` function use them::

   from distutils.command.build_py import build_py as _build_py

   from distutils.core import setup

   class build_py(_build_py):

       """Specialized Python source builder."""

       # implement whatever needs to be different...

   setup(cmdclass={'build_py': build_py},

         ...)

This approach is most valuable if the new implementations must be used to use a

particular package, as everyone interested in the package will need to have the

new command implementation.

Beginning with Python 2.4, a third option is available, intended to allow new

commands to be added which can support existing :file:`setup.py` scripts without

requiring modifications to the Python installation.  This is expected to allow

third-party extensions to provide support for additional packaging systems, but

the commands can be used for anything distutils commands can be used for.  A new

configuration option, :option:`command_packages` (command-line option

:option:`--command-packages`), can be used to specify additional packages to be

searched for modules implementing commands.  Like all distutils options, this

can be specified on the command line or in a configuration file.  This option

can only be set in the ``[global]`` section of a configuration file, or before

any commands on the command line.  If set in a configuration file, it can be

overridden from the command line; setting it to an empty string on the command

line causes the default to be used.  This should never be set in a configuration

file provided with a package.

This new option can be used to add any number of packages to the list of

packages searched for command implementations; multiple package names should be

separated by commas.  When not specified, the search is only performed in the

:mod:`distutils.command` package.  When :file:`setup.py` is run with the option

:option:`--command-packages` :option:`distcmds,buildcmds`, however, the packages

:mod:`distutils.command`, :mod:`distcmds`, and :mod:`buildcmds` will be searched

in that order.  New commands are expected to be implemented in modules of the

same name as the command by classes sharing the same name.  Given the example

command line option above, the command :command:`bdist_openpkg` could be

implemented by the class :class:`distcmds.bdist_openpkg.bdist_openpkg` or

:class:`buildcmds.bdist_openpkg.bdist_openpkg`.

Adding new distribution types

=============================

Commands that create distributions (files in the :file:`dist/` directory) need

to add ``(command, filename)`` pairs to ``self.distribution.dist_files`` so that

:command:`upload` can upload it to PyPI.  The *filename* in the pair contains no

path information, only the name of the file itself.  In dry-run mode, pairs

should still be added to represent what would have been created.