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.


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
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.

class setuptools.Command(dist: Distribution, **kw)

Setuptools internal actions are organized using a command design pattern. This means that each action (or group of closely related actions) executed during the build should be implemented as a Command subclass.

These commands are abstractions and do not necessarily correspond to a command that can (or should) be executed via a terminal, in a CLI fashion (although historically they would).

When creating a new command from scratch, custom defined classes SHOULD inherit from setuptools.Command and implement a few mandatory methods. Between these mandatory methods, are listed:


Set or (reset) all options/attributes/caches used by the command to their default values. Note that these values may be overwritten during the build.


Set final values for all options/attributes used by the command. Most of the time, each option/attribute/cache should only be set if it does not have any value yet (e.g. if self.attr is None: self.attr = val).


Execute the actions intended by the command. (Side effects SHOULD only take place when run is executed, for example, creating new files or writing to the terminal output).

A useful analogy for command classes is to think of them as subroutines with local variables called “options”. The options are “declared” in initialize_options() and “defined” (given their final values, aka “finalized”) in finalize_options(), both of which must be defined by every command class. The “body” of the subroutine, (where it does all the work) is the run() method. Between initialize_options() and finalize_options(), setuptools may set the values for options/attributes based on user’s input (or circumstance), which means that the implementation should be careful to not overwrite values in finalize_options unless necessary.

Please note that other commands (or other parts of setuptools) may also overwrite the values of the command’s options/attributes multiple times during the build process. Therefore it is important to consistently implement initialize_options() and finalize_options(). For example, all derived attributes (or attributes that depend on the value of other attributes) SHOULD be recomputed in finalize_options.

When overwriting existing commands, custom defined classes MUST abide by the same APIs implemented by the original class. They also SHOULD inherit from the original class.

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:

class setuptools.command.build.SubCommand(*args, **kwargs)

In order to support editable installations (see PEP 660) all build subcommands SHOULD implement this protocol. They also MUST inherit from setuptools.Command.

When creating an editable wheel, setuptools will try to evaluate custom build subcommands using the following procedure:

  1. setuptools will set the editable_mode attribute to True

  2. setuptools will execute the run() command.


    Subcommands SHOULD take advantage of editable_mode=True to adequate its behaviour or perform optimisations.

    For example, if a subcommand doesn’t need to generate an extra file and all it does is to copy a source file into the build directory, run() SHOULD simply “early return”.

    Similarly, if the subcommand creates files that would be placed alongside Python files in the final distribution, during an editable install the command SHOULD generate these files “in place” (i.e. write them to the original source directory, instead of using the build directory). Note that get_output_mapping() should reflect that and include mappings for “in place” builds accordingly.

  3. setuptools use any knowledge it can derive from the return values of get_outputs() and get_output_mapping() to create an editable wheel. When relevant setuptools MAY attempt to use file links based on the value of get_output_mapping(). Alternatively, setuptools MAY attempt to use import hooks to redirect any attempt to import to the directory with the original source code and other files built in place.

Please note that custom sub-commands SHOULD NOT rely on run() being executed (or not) to provide correct return values for get_outputs(), get_output_mapping() or get_source_files(). The get_* methods should work independently of run().

build_lib: str

String representing the directory where the build artifacts should be stored, e.g. build/lib. For example, if a distribution wants to provide a Python module named pkg.mod, then a corresponding file should be written to {build_lib}/package/module.py. A way of thinking about this is that the files saved under build_lib would be eventually copied to one of the directories in site.PREFIXES upon installation.

A command that produces platform-independent files (e.g. compiling text templates into Python functions), CAN initialize build_lib by copying its value from the build_py command. On the other hand, a command that produces platform-specific files CAN initialize build_lib by copying its value from the build_ext command. In general this is done inside the finalize_options method with the help of the set_undefined_options command:

def finalize_options(self):
    self.set_undefined_options("build_py", ("build_lib", "build_lib"))
editable_mode: bool = False

Boolean flag that will be set to True when setuptools is used for an editable installation (see PEP 660). Implementations SHOULD explicitly set the default value of this attribute to False. When subcommands run, they can use this flag to perform optimizations or change their behaviour accordingly.


(Required by the original setuptools.Command interface)

get_output_mapping() dict[str, str]

Return a mapping between destination files as they would be produced by the build (dict keys) into the respective existing (source) files (dict values). Existing (source) files should be represented as strings relative to the project root directory. Destination files should be strings in the form of "{build_lib}/destination/file/path".

get_outputs() list[str]

Return a list of files intended for distribution as they would have been produced by the build. These files should be strings in the form of "{build_lib}/destination/file/path".


The return value of get_output() should include all files used as keys in get_output_mapping() plus files that are generated during the build and don’t correspond to any source file already present in the project.

get_source_files() list[str]

Return a list of all files that are used by the command to create the expected outputs. For example, if your build command transpiles Java files into Python, you should list here all the Java files. The primary purpose of this function is to help populating the sdist with all the files necessary to build the distribution. All files should be strings relative to the project root directory.


(Required by the original setuptools.Command interface)


(Required by the original setuptools.Command interface)

Adding Arguments


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
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(
            "%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 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
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:

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

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.