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# 9. 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 `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 `.py` files as a convenience.

Most distutils command implementations are subclasses of the `Command` class from `distutils.cmd`. New commands may directly inherit from `Command`, while replacements often derive from `Command` indirectly, directly subclassing the command they are replacing. Commands are required to derive from `Command`.

## 9.1. 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 `setup.py` script, and cause the `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 `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, command_packages (command-line 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 `distutils.command` package. When `setup.py` is run with the option --command-packages distcmds,buildcmds, however, the packages `distutils.command`, `distcmds`, and `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 bdist_openpkg could be implemented by the class `distcmds.bdist_openpkg.bdist_openpkg` or `buildcmds.bdist_openpkg.bdist_openpkg`.

## 9.2. Adding new distribution types

Commands that create distributions (files in the `dist/` directory) need to add `(command, filename)` pairs to `self.distribution.dist_files` so that 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.

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