1 .. _extending-distutils:

     2 

     3 *******************

     4 Extending Distutils

     5 *******************

     6 

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

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

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

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

    11 command operates on a package.

    12 

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

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

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

    16 convenience.

    17 

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

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

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

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

    22 required to derive from :class:`Command`.

    23 

    24 .. % \section{Extending existing commands}

    25 .. % \label{extend-existing}

    26 

    27 .. % \section{Writing new commands}

    28 .. % \label{new-commands}

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

    30 

    31 

    32 Integrating new commands

    33 ========================

    34 

    35 There are different ways to integrate new command implementations into

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

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

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

    39 

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

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

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

    43 

    44    from distutils.command.build_py import build_py as _build_py

    45    from distutils.core import setup

    46 

    47    class build_py(_build_py):

    48        """Specialized Python source builder."""

    49 

    50        # implement whatever needs to be different...

    51 

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

    53          ...)

    54 

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

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

    57 new command implementation.

    58 

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

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

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

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

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

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

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

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

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

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

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

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

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

    72 file provided with a package.

    73 

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

    75 packages searched for command implementations; multiple package names should be

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

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

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

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

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

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

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

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

    84 :class:`buildcmds.bdist_openpkg.bdist_openpkg`.

    85 

    86 

    87 Adding new distribution types

    88 =============================

    89 

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

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

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

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

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

    95 

    96