Specifying external dependencies in pyproject.toml

[BlueMagma2]

Introduction

There currently exists a draft proposing a way to declare external dependencies in pyproject,toml: https://peps.python.org/pep-0725/

For example in some of my project I specify the following section in my pyproject.toml:

[external]
dependencies = ["pkg:generic/ffmpeg"]

What I expect this will do is add a line in the PKG-INFO file of my build packages as specified here: https://packaging.python.org/en/latest/specifications/core-metadata/#requires-external-multiple-use

Requires-External: pkg:generic/ffmpeg

Currently no build system properly handle this. Since I was using poetry I made a poetry plugin that takes care of detecting this section and adding the relevant line in PKG-INFO during build: https://pypi.org/project/poetry-external-dependencies/

Then when deploying my package to different system I use a script that detect the external dependency and install what's required on the host system, in the example case it would automatically install ffmpeg using apt or apk or other package manager the system offer. Something like the following:

import warnings
from importlib.metadata import metadata

warnings.filterwarnings('ignore', category=DeprecationWarning)

d = {dist.project_name: metadata(dist.project_name).keys() for dist in __import__('pkg_resources').working_set}
requires_external = [metadata(key)['Requires-External'] for key, meta in d.items() if 'Requires-External' in meta]
included_external = [req.split('/')[-1] for req in requires_external if req.startswith('pkg:generic/')]

if included_external:
    print(f"apt install -y {' '.join(included_external)}")  # noqa: T201
else:
    print('echo "No python \'Requires-External\' found."')  # noqa: T201

The question

I've started to transition my package to uv. How can I have the Requires-External line added to the PGK-INFO at build time ? Does uv support some form of plugin ? If not what can I do to have it work ?

[BlueMagma2]

I just now realise that currently uv doesn't have a build backend and uses hatchling, this question might be better suited to be asked on hatchling's repository. If I find out how to solve my problem though there I'll report back here for reference

[BlueMagma2]

In case anyone is wondering how to do this, I solve my issue using a hatchling build hook, I might turn this into a plugin if I get the time:

from copy import deepcopy

from hatchling.builders.hooks.plugin.interface import BuildHookInterface
from hatchling.metadata.core import ProjectMetadata

class CustomBuildHook(BuildHookInterface):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def initialize(self, version: str, build_data: dict[str, any]) -> None:
        self._default_constructor = self.build_config.core_metadata_constructor

        def metadata_constructor_extended(local_self, metadata: ProjectMetadata, extra_dependencies: tuple[str] | None = None) -> str:
            metadata_file = self._default_constructor(metadata, extra_dependencies)

            external_dependencies = None
            if 'external-dependencies' in metadata.core.config:
                external_dependencies = deepcopy(metadata.core.config['external-dependencies'])

            elif 'external' in metadata.config and 'dependencies' in metadata.config['external']:
                external_dependencies = deepcopy(metadata.config['external']['dependencies'])

            if external_dependencies:
                header_section = metadata_file
                content_section = ''
                if 'Description-Content-Type' in metadata_file:
                    split_file = metadata_file.split('Description-Content-Type')
                    header_section = split_file[0]
                    content_section = split_file[1]
                
                print(f"  - {self.PLUGIN_NAME}")
                for dependency in external_dependencies:
                    print(f"    - Requires-External: {dependency}")
                    header_section += f'Requires-External: {dependency}\n'
                metadata_file = header_section +'Description-Content-Type' + content_section
            
            return metadata_file

        type(self.build_config).core_metadata_constructor = metadata_constructor_extended

[zanieb]

Cool thanks for following up on your issue! We're interested in a build backend (#3957) so we may tackle this eventually.

cc @ofek

[BlueMagma2]

For reference, I've made it into a plugin for this: https://pypi.org/project/hatch-external-dependencies/