# Extending or Customizing Setuptools

Setuptools design is based on the distutils package originally distributed as part of Python’s standard library, effectively serving as its successor (as established in PEP 632).

This means that `setuptools` strives to honor the extension mechanisms provided by `distutils`, and allows developers to create third party packages that modify or augment the build process behavior.

A simple way of doing that is to hook in new or existing 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 Advertising Behavior for some more background on entry points).

The following sections describe the most common procedures for extending the `distutils` functionality used by `setuptools`.

Important

Any entry-point defined in your `setup.cfg`, `setup.py` or `pyproject.toml` files are not immediately available for use. Your package needs to be installed first, then `setuptools` will be able to access these entry points. For example consider a `Project-A` that defines entry points. When building `Project-A`, these will not be available. If `Project-B` declares a build system requirement on `Project-A`, then `setuptools` will be able to use `Project-A`’ customizations.

## Customizing Commands

Both `setuptools` and `distutils` are structured around the command design pattern. This means that each main action executed when building a distribution package (such as creating a sdist or wheel) correspond to the implementation of a Python class.

Originally in `distutils`, these commands would correspond to actual CLI arguments that could be passed to the `setup.py` script to trigger a different aspect of the build. In `setuptools`, however, these command objects are just a design abstraction that encapsulate logic and help to organise the code.

You can overwrite existing commands (or add new ones) 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 project:

    # setup.cfg
    ...
    [options.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 (documented below).

Once a project containing such entry points has been activated on `sys.path`, (e.g. by running `pip install`) the command(s) will be available to any `setuptools`-based project. In fact, this is how setuptools’ own commands are installed: the setuptools project’s setup script defines entry points for them!

The commands `sdist`, `build_py` and `build_ext` are especially useful to customize `setuptools` builds. Note however that when overwriting existing commands, you should be very careful to maintain API compatibility. Custom commands should try to replicate the same overall behavior as the original classes, and when possible, even inherit from them.

You should also consider handling exceptions such as `CompileError`, `LinkError`, `LibError`, among others. These exceptions are available in the `setuptools.errors` module.

### Supporting sdists and editable installs in `build` sub-commands

`build` sub-commands (like `build_py` and `build_ext`) are encouraged to implement the following protocol:

## Adding 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 extension project:

    # setup.cfg
    ...
    [options.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 SetupError(
                f"{attr!r} must be a boolean value (got {value!r}"
            )

Your function should accept three arguments: the `Distribution` object, the attribute name, and the attribute value. It should raise a `SetupError` (from the `setuptools.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 extensions to define a common argument, as long as they agree on what values of that argument are valid.

## 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.

## Defining Additional Metadata

Some extensible applications and frameworks may need to define their own kinds of metadata, which they can then access using the `importlib.metadata` APIs. Ordinarily, this is done by having plugin developers include additional files in their `ProjectName.egg-info` directory. However, since it can be tedious to create such files by hand, you may want to create an extension that will create the necessary files from arguments to `setup()`, in much the same way that `setuptools` does for many of the `setup()` arguments it adds. See the section below for more details.

### 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 an 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 an 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.cfg
    ...
    [options.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 `logging` 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

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

- SVN: 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:

    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:

    # setup.cfg
    ...

    [options.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 `logging.warning()` to inform the user of the missing program(s).

## Final Remarks

- To use a `setuptools` plugin, your users will need to add your package as a build requirement to their build-system configuration. Please check out our guides on Dependencies Management in Setuptools for more information.

- Directly calling `python setup.py ...` is considered a deprecated practice. You should not add new commands to `setuptools` expecting them to be run via this interface.