Edit File: extension.rst
.. _Creating ``distutils`` Extensions: Creating ``distutils`` Extensions ================================= It can be hard to add new commands or setup arguments to the distutils. But the ``setuptools`` package makes it a bit easier, by allowing you to distribute a distutils extension as a separate project, and then have projects that need the extension just refer to it in their ``setup_requires`` argument. With ``setuptools``, your distutils extension projects can hook in new commands and ``setup()`` arguments just by defining "entry points". These are mappings from command or argument names to a specification of where to import a handler from. (See the section on :ref:`Dynamic Discovery of Services and Plugins` above for some more background on entry points.) Adding Commands --------------- You can add new ``setup`` commands by defining entry points in the ``distutils.commands`` group. For example, if you wanted to add a ``foo`` command, you might add something like this to your distutils extension project's setup script:: setup( # ... entry_points={ "distutils.commands": [ "foo = mypackage.some_module:foo", ], }, ) (Assuming, of course, that the ``foo`` class in ``mypackage.some_module`` is a ``setuptools.Command`` subclass.) Once a project containing such entry points has been activated on ``sys.path``, (e.g. by running "install" or "develop" with a site-packages installation directory) the command(s) will be available to any ``setuptools``-based setup scripts. It is not necessary to use the ``--command-packages`` option or to monkeypatch the ``distutils.command`` package to install your commands; ``setuptools`` automatically adds a wrapper to the distutils to search for entry points in the active distributions on ``sys.path``. In fact, this is how setuptools' own commands are installed: the setuptools project's setup script defines entry points for them! Adding ``setup()`` Arguments ---------------------------- .. warning:: Adding arguments to setup is discouraged as such arguments are only supported through imperative execution and not supported through declarative config. Sometimes, your commands may need additional arguments to the ``setup()`` call. You can enable this by defining entry points in the ``distutils.setup_keywords`` group. For example, if you wanted a ``setup()`` argument called ``bar_baz``, you might add something like this to your distutils extension project's setup script:: setup( # ... entry_points={ "distutils.commands": [ "foo = mypackage.some_module:foo", ], "distutils.setup_keywords": [ "bar_baz = mypackage.some_module:validate_bar_baz", ], }, ) The idea here is that the entry point defines a function that will be called to validate the ``setup()`` argument, if it's supplied. The ``Distribution`` object will have the initial value of the attribute set to ``None``, and the validation function will only be called if the ``setup()`` call sets it to a non-None value. Here's an example validation function:: def assert_bool(dist, attr, value): """Verify that value is True, False, 0, or 1""" if bool(value) != value: raise DistutilsSetupError( "%r must be a boolean value (got %r)" % (attr,value) ) Your function should accept three arguments: the ``Distribution`` object, the attribute name, and the attribute value. It should raise a ``DistutilsSetupError`` (from the ``distutils.errors`` module) if the argument is invalid. Remember, your function will only be called with non-None values, and the default value of arguments defined this way is always None. So, your commands should always be prepared for the possibility that the attribute will be ``None`` when they access it later. If more than one active distribution defines an entry point for the same ``setup()`` argument, *all* of them will be called. This allows multiple distutils extensions to define a common argument, as long as they agree on what values of that argument are valid. Also note that as with commands, it is not necessary to subclass or monkeypatch the distutils ``Distribution`` class in order to add your arguments; it is sufficient to define the entry points in your extension, as long as any setup script using your extension lists your project in its ``setup_requires`` argument. Customizing Distribution Options -------------------------------- Plugins may wish to extend or alter the options on a Distribution object to suit the purposes of that project. For example, a tool that infers the ``Distribution.version`` from SCM-metadata may need to hook into the option finalization. To enable this feature, Setuptools offers an entry point "setuptools.finalize_distribution_options". That entry point must be a callable taking one argument (the Distribution instance). If the callable has an ``.order`` property, that value will be used to determine the order in which the hook is called. Lower numbers are called first and the default is zero (0). Plugins may read, alter, and set properties on the distribution, but each plugin is encouraged to load the configuration/settings for their behavior independently. .. _Adding new EGG-INFO Files: Adding new EGG-INFO Files ------------------------- Some extensible applications or frameworks may want to allow third parties to develop plugins with application or framework-specific metadata included in the plugins' EGG-INFO directory, for easy access via the ``pkg_resources`` metadata API. The easiest way to allow this is to create a distutils extension to be used from the plugin projects' setup scripts (via ``setup_requires``) that defines a new setup keyword, and then uses that data to write an EGG-INFO file when the ``egg_info`` command is run. The ``egg_info`` command looks for extension points in an ``egg_info.writers`` group, and calls them to write the files. Here's a simple example of a distutils extension defining a setup argument ``foo_bar``, which is a list of lines that will be written to ``foo_bar.txt`` in the EGG-INFO directory of any project that uses the argument:: setup( # ... entry_points={ "distutils.setup_keywords": [ "foo_bar = setuptools.dist:assert_string_list", ], "egg_info.writers": [ "foo_bar.txt = setuptools.command.egg_info:write_arg", ], }, ) This simple example makes use of two utility functions defined by setuptools for its own use: a routine to validate that a setup keyword is a sequence of strings, and another one that looks up a setup argument and writes it to a file. Here's what the writer utility looks like:: def write_arg(cmd, basename, filename): argname = os.path.splitext(basename)[0] value = getattr(cmd.distribution, argname, None) if value is not None: value = "\n".join(value) + "\n" cmd.write_or_delete_file(argname, filename, value) As you can see, ``egg_info.writers`` entry points must be a function taking three arguments: a ``egg_info`` command instance, the basename of the file to write (e.g. ``foo_bar.txt``), and the actual full filename that should be written to. In general, writer functions should honor the command object's ``dry_run`` setting when writing files, and use the ``distutils.log`` object to do any console output. The easiest way to conform to this requirement is to use the ``cmd`` object's ``write_file()``, ``delete_file()``, and ``write_or_delete_file()`` methods exclusively for your file operations. See those methods' docstrings for more details. .. _Adding Support for Revision Control Systems: Adding Support for Revision Control Systems ------------------------------------------------- If the files you want to include in the source distribution are tracked using Git, Mercurial or SVN, you can use the following packages to achieve that: - Git and Mercurial: `setuptools_scm <https://pypi.org/project/setuptools_scm/>`_ - SVN: `setuptools_svn <https://pypi.org/project/setuptools_svn/>`_ If you would like to create a plugin for ``setuptools`` to find files tracked by another revision control system, you can do so by adding an entry point to the ``setuptools.file_finders`` group. The entry point should be a function accepting a single directory name, and should yield all the filenames within that directory (and any subdirectories thereof) that are under revision control. For example, if you were going to create a plugin for a revision control system called "foobar", you would write a function something like this: .. code-block:: python def find_files_for_foobar(dirname): ... # loop to yield paths that start with `dirname` And you would register it in a setup script using something like this:: entry_points={ "setuptools.file_finders": [ "foobar = my_foobar_module:find_files_for_foobar", ] } Then, anyone who wants to use your plugin can simply install it, and their local setuptools installation will be able to find the necessary files. It is not necessary to distribute source control plugins with projects that simply use the other source control system, or to specify the plugins in ``setup_requires``. When you create a source distribution with the ``sdist`` command, setuptools automatically records what files were found in the ``SOURCES.txt`` file. That way, recipients of source distributions don't need to have revision control at all. However, if someone is working on a package by checking out with that system, they will need the same plugin(s) that the original author is using. A few important points for writing revision control file finders: * Your finder function MUST return relative paths, created by appending to the passed-in directory name. Absolute paths are NOT allowed, nor are relative paths that reference a parent directory of the passed-in directory. * Your finder function MUST accept an empty string as the directory name, meaning the current directory. You MUST NOT convert this to a dot; just yield relative paths. So, yielding a subdirectory named ``some/dir`` under the current directory should NOT be rendered as ``./some/dir`` or ``/somewhere/some/dir``, but *always* as simply ``some/dir`` * Your finder function SHOULD NOT raise any errors, and SHOULD deal gracefully with the absence of needed programs (i.e., ones belonging to the revision control system itself. It *may*, however, use ``distutils.log.warn()`` to inform the user of the missing program(s).