Custom `setup_py()` kwargs

By default, Pants will simply copy the kwargs (keyword arguments) used in the provides=setup_py() field in the BUILD file when running the package goal on a python_distribution.

You can instead write a plugin to add your own logic to what kwargs are used for the setup() function to do any of these things:

Reduce boilerplate by hardcoding common kwargs.
Read from the file system to dynamically determine kwargs, such as the long_description or version.
Run processes like git to dynamically determine kwargs like version.

Note: regardless of if you wrote a plugin or not, Pants will automatically set some kwargs like install_requires and namespace_packages based on analyzing your code.

Note: there may only be at most one applicable plugin per target customizing the kwargs for the setup() function.

📘
Example
See here for an example that Pants uses internally for its python_distribution targets. This plugin demonstrates reading from the file system to set the version and long_description kwargs, along with adding hardcoded kwargs.

1. Set up a subclass of `SetupKwargsRequest`

Set the class method is_applicable() to determine whether your implementation should be used for the particular python_distribution target. If False, Pants will use the default implementation which simply uses the explicitly provided setup_py from the BUILD file.

In this example, we will always use our custom implementation:

from pants.backend.python.goals.setup_py import SetupKwargsRequest
from pants.engine.target import Target

class CustomSetupKwargsRequest(SetupKwargsRequest):
    @classmethod
    def is_applicable(cls, _: Target) -> bool:
        return True

This example will only use our plugin implementation for python_distribution targets defined in the folder src/python/project1.

class CustomSetupKwargsRequest(SetupKwargsRequest):
    @classmethod
    def is_applicable(cls, target: Target) -> bool:
        return target.address.spec.startswith("src/python/project1")

Then, register your new SetupKwargsRequest with a UnionRule so that Pants knows your implementation exists:

from pants.engine.rules import collect_rules
from pants.engine.unions import UnionRule

...

def rules():
    return [
      	*collect_rules(),
        UnionRule(SetupKwargsRequest, CustomSetupKwargsRequest),
    ]

📘
Consider defining custom python_distribution target types
If you don't want to always use a single custom implementation, an effective approach could be to create custom python_distribution target types so that your users decide which implementation they want to use in their BUILD files.
For example, a user could do this:
pants_python_distribution(
   name="my-dist",
   dependencies=[...],
   provides=setup_py(...)
)

pants_contrib_python_distribution(
   name="my-dist",
   dependencies=[...],
   provides=setup_py(...)
)
To support this workflow, create new target types.
class PantsPythonDistribution(Target):
   alias = "pants_python_distribution"
   core_fields = PythonDistribution.core_fields

class PantsContribPythonDistribution(Target):
   alias = "pants_contrib_python_distribution"
   core_fields = PythonDistribution.core_fields
Then, for each SetupKwargsRequest subclass, check which target type was used:
class PantsSetupKwargsRequest(SetupKwargsRequest):
    @classmethod
    def is_applicable(cls, target: Target) -> bool:
        return isinstance(target, PantsPythonDistribution)

2. Create a rule with your logic

Your rule should take as a parameter the SetupKwargsRequest from step 1. This type has two fields: target: Target and explicit_kwargs: dict[str, Any]. You can use these fields to get more information on the target you are generating a setup.py for.

Your rule should return SetupKwargs, which takes two arguments: kwargs: dict[str, Any] and address: Address.

For example, this will simply hardcode a kwarg:

from pants.backend.python.goals.setup_py import SetupKwargs
from pants.engine.rules import rule

@rule
async def setup_kwargs_plugin(request: CustomSetupKwargsRequest) -> SetupKwargs:
    return SetupKwargs(
        {**request.explicit_kwargs, "plugin_demo": "hello world"}, address=request.target.address
    )

Update your plugin's register.py to activate this file's rules.

from python_plugins import custom_setup_py

def rules():
   return custom_setup_py.rules()

Then, run ./pants package path/to:python_distribution and inspect the generated setup.pyto confirm that your plugin worked correctly.

Often, you will want to read from a file in your project to set kwargs like version or long_description. Use await Get(DigestContents, PathGlobs) to do this (see File system):

from pants.backend.python.goals.setup_py import SetupKwargs
from pants.engine.fs import DigestContents, GlobMatchErrorBehavior, PathGlobs
from pants.engine.rules import rule

@rule
async def setup_kwargs_plugin(request: CustomSetupKwargsRequest) -> SetupKwargs:
    digest_contents = await Get(
        DigestContents,
        PathGlobs(
            ["project/ABOUT.rst"],
            description_of_origin="`setup_py()` plugin",
            glob_match_error_behavior=GlobMatchErrorBehavior.error,
        ),
    )
    about_page_content = digest_contents[0].content.decode()
    return SetupKwargs(
        {**request.explicit_kwargs, "long_description": "\n".join(about_page_content)},
        address=request.target.address
    )

It can be helpful to allow users to add additional kwargs to their BUILD files for you to consume in your plugin. For example, this plugin adds a custom long_description_path field, which gets popped and replaced by the plugin with a normalized long_description kwarg:

python_distribution(
    name="mydist",
    dependencies=[...],
    provides=setup_py(
        name="mydist",
        ...
        long_description_path="README.md",
    ),
    setup_py_commands=["sdist"]
)

import os.path

from pants.backend.python.goals.setup_py import SetupKwargs
from pants.engine.fs import DigestContents, GlobMatchErrorBehavior, PathGlobs
from pants.engine.rules import rule

@rule
async def setup_kwargs_plugin(request: CustomSetupKwargsRequest) -> SetupKwargs:
    original_kwargs = request.explicit_kwargs.copy()
    long_description_relpath = original_kwargs.pop("long_description_file", None)
    if not long_description_relpath:
        raise ValueError(
            f"The python_distribution target {request.target.address} did not include "
            "`long_description_file` in its setup_py's kwargs. Our plugin requires this! "
            "Please set to a path relative to the BUILD file, e.g. `ABOUT.md`."
        )

    build_file_path = request.target.address.spec_path
    long_description_path = os.path.join(build_file_path, long_description_relpath)
    digest_contents = await Get(
        DigestContents,
        PathGlobs(
            [long_description_path],
            description_of_origin=f"the 'long_description_file' kwarg in {request.target.address}",
            glob_match_error_behavior=GlobMatchErrorBehavior.error,
        ),
    )
    description_content = digest_contents[0].content.decode()
    return SetupKwargs(
        {**original_kwargs, "long_description": "\n".join(description_content)},
        address=request.target.address
    )

Refer to these guides for additional things you may want to do in your plugin:

Read from options. Also see here for an example.
Read values from the target using the Target API.
Run a Process, such as git. Also see Installing tools.

1. Set up a subclass of SetupKwargsRequest

2. Create a rule with your logic

1. Set up a subclass of `SetupKwargsRequest`