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+.. _extending-packaging:
+
+*******************
+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 packaging 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 packaging command implementations are subclasses of the
+:class:`packaging.cmd.Command` class.  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`.
+
+.. .. _extend-existing:
+      Extending existing commands
+      ===========================
+
+
+.. .. _new-commands:
+      Writing new commands
+      ====================
+
+
+Integrating new commands
+========================
+
+There are different ways to integrate new command implementations into
+packaging.  The most difficult is to lobby for the inclusion of the new features
+in packaging 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:`packaging.core.setup` function use them::
+
+   from packaging.core import setup
+   from packaging.command.build_py import build_py as _build_py
+
+   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 packaging 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 packaging 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:`packaging.command` package.  When :file:`setup.py` is run with the option
+:option:`--command-packages` :option:`distcmds,buildcmds`, however, the packages
+:mod:`packaging.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.