PEP: 631
Title: Dependency specification in pyproject.toml based on PEP 508
Author: Ofek Lev <ofekmeister@gmail.com>
Sponsor: Paul Ganssle <paul@ganssle.io>
Discussions-To: https://discuss.python.org/t/5018
Status: Accepted
Type: Standards Track
Content-Type: text/x-rst
Created: 20-Aug-2020
Post-History: 20-Aug-2020
Resolution: https://discuss.python.org/t/how-to-specify-dependencies-pep-508-strings-or-a-table-in-toml/5243/38

Abstract
========

This PEP specifies how to write a project's dependencies in a
``pyproject.toml`` file for packaging-related tools to consume
using the `fields defined in PEP 621`_.

.. note::
    This PEP has been accepted and is expected to be merged into
    :pep:`621`.

Entries
=======

All dependency entries MUST be valid `PEP 508 strings`_.

Build backends SHOULD abort at load time for any parsing errors.

::

    from packaging.requirements import InvalidRequirement, Requirement

    ...

    try:
        Requirement(entry)
    except InvalidRequirement:
        # exit

Specification
=============

dependencies
------------

- Format: array of strings
- Related core metadata:

  - `Requires-Dist`_

Every element MUST be an `entry <#entries>`_.

::

    [project]
    dependencies = [
      'PyYAML ~= 5.0',
      'requests[security] < 3',
      'subprocess32; python_version < "3.2"',
    ]

optional-dependencies
---------------------

- Format: table
- Related core metadata:

  - `Provides-Extra`_
  - `Requires-Dist`_

Each key is the name of the provided option, with each value being the same type as
the `dependencies <#dependencies>`_ field i.e. an array of strings.

::

    [project.optional-dependencies]
    tests = [
      'coverage>=5.0.3',
      'pytest',
      'pytest-benchmark[histogram]>=3.2.1',
    ]

Example
=======

This is a real-world example port of what `docker-compose`_ defines.

::

    [project]
    dependencies = [
      'cached-property >= 1.2.0, < 2',
      'distro >= 1.5.0, < 2',
      'docker[ssh] >= 4.2.2, < 5',
      'dockerpty >= 0.4.1, < 1',
      'docopt >= 0.6.1, < 1',
      'jsonschema >= 2.5.1, < 4',
      'PyYAML >= 3.10, < 6',
      'python-dotenv >= 0.13.0, < 1',
      'requests >= 2.20.0, < 3',
      'texttable >= 0.9.0, < 2',
      'websocket-client >= 0.32.0, < 1',

      # Conditional
      'backports.shutil_get_terminal_size == 1.0.0; python_version < "3.3"',
      'backports.ssl_match_hostname >= 3.5, < 4; python_version < "3.5"',
      'colorama >= 0.4, < 1; sys_platform == "win32"',
      'enum34 >= 1.0.4, < 2; python_version < "3.4"',
      'ipaddress >= 1.0.16, < 2; python_version < "3.3"',
      'subprocess32 >= 3.5.4, < 4; python_version < "3.2"',
    ]

    [project.optional-dependencies]
    socks = [ 'PySocks >= 1.5.6, != 1.5.7, < 2' ]
    tests = [
      'ddt >= 1.2.2, < 2',
      'pytest < 6',
      'mock >= 1.0.1, < 4; python_version < "3.4"',
    ]

Implementation
==============

Parsing
-------

::

    from packaging.requirements import InvalidRequirement, Requirement

    def parse_dependencies(config):
        dependencies = config.get('dependencies', [])
        if not isinstance(dependencies, list):
            raise TypeError('Field `project.dependencies` must be an array')

        for i, entry in enumerate(dependencies, 1):
            if not isinstance(entry, str):
                raise TypeError(f'Dependency #{i} of field `project.dependencies` must be a string')

            try:
                Requirement(entry)
            except InvalidRequirement as e:
                raise ValueError(f'Dependency #{i} of field `project.dependencies` is invalid: {e}')

        return dependencies

    def parse_optional_dependencies(config):
        optional_dependencies = config.get('optional-dependencies', {})
        if not isinstance(optional_dependencies, dict):
            raise TypeError('Field `project.optional-dependencies` must be a table')

        optional_dependency_entries = {}

        for option, dependencies in optional_dependencies.items():
            if not isinstance(dependencies, list):
                raise TypeError(
                    f'Dependencies for option `{option}` of field '
                    '`project.optional-dependencies` must be an array'
                )

            entries = []

            for i, entry in enumerate(dependencies, 1):
                if not isinstance(entry, str):
                    raise TypeError(
                        f'Dependency #{i} of option `{option}` of field '
                        '`project.optional-dependencies` must be a string'
                    )

                try:
                    Requirement(entry)
                except InvalidRequirement as e:
                    raise ValueError(
                        f'Dependency #{i} of option `{option}` of field '
                        f'`project.optional-dependencies` is invalid: {e}'
                    )
                else:
                    entries.append(entry)

            optional_dependency_entries[option] = entries

        return optional_dependency_entries

Metadata
--------

::

    def construct_metadata_file(metadata_object):
        """
        https://packaging.python.org/specifications/core-metadata/
        """
        metadata_file = 'Metadata-Version: 2.1\n'

        ...

        if metadata_object.dependencies:
            # Sort dependencies to ensure reproducible builds
            for dependency in sorted(metadata_object.dependencies):
                metadata_file += f'Requires-Dist: {dependency}\n'

        if metadata_object.optional_dependencies:
            # Sort extras and dependencies to ensure reproducible builds
            for option, dependencies in sorted(metadata_object.optional_dependencies.items()):
                metadata_file += f'Provides-Extra: {option}\n'
                for dependency in sorted(dependencies):
                    if ';' in dependency:
                        metadata_file += f'Requires-Dist: {dependency} and extra == "{option}"\n'
                    else:
                        metadata_file += f'Requires-Dist: {dependency}; extra == "{option}"\n'

        ...

        return metadata_file

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.


.. _fields defined in PEP 621: https://www.python.org/dev/peps/pep-0621/#dependencies-optional-dependencies
.. _PEP 508 strings: https://www.python.org/dev/peps/pep-0508/
.. _Requires-Dist: https://packaging.python.org/specifications/core-metadata/#requires-dist-multiple-use
.. _Provides-Extra: https://packaging.python.org/specifications/core-metadata/#provides-extra-multiple-use
.. _docker-compose: https://github.com/docker/compose/blob/789bfb0e8b2e61f15f423d371508b698c64b057f/setup.py#L28-L61

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