python-peps/peps/pep-0735.rst

PEP: 735
Title: Dependency Groups in pyproject.toml
Author: Stephen Rosen <sirosen0@gmail.com>
Sponsor: Brett Cannon <brett@python.org>
PEP-Delegate: Paul Moore <p.f.moore@gmail.com>
Discussions-To: https://discuss.python.org/t/39233
Status: Draft
Type: Standards Track
Topic: Packaging
Created: 20-Nov-2023
Post-History: `14-Nov-2023 <https://discuss.python.org/t/29684>`__, `20-Nov-2023 <https://discuss.python.org/t/39233>`__

Abstract
========

This PEP specifies a mechanism for storing package requirements in
``pyproject.toml`` files such that they are not included in any built distribution of
the project.

This is suitable for creating named groups of dependencies, similar to
``requirements.txt`` files, which launchers, IDEs, and other tools can find and
identify by name.

The feature defined here is referred to as "Dependency Groups".

Motivation
==========

There are two major use cases for which the Python community has no
standardized answer:

* How should development dependencies be defined for packages?

* How should dependencies be defined for projects which do not build
  distributions (non-package projects)?

In support of these two needs, there are two common solutions which are similar
to this proposal:

* ``requirements.txt`` files

* package `extras <https://packaging.python.org/en/latest/specifications/dependency-specifiers/#extras>`__

Both ``requirements.txt`` files and ``extras`` have limitations which this
standard seeks to overcome.

Note that the two use cases above describe two different types of projects
which this PEP seeks to support:

* Python packages, such as libraries

* non-package projects, such as data science projects

Limitations of ``requirements.txt`` files
-----------------------------------------

Many projects may define one or more ``requirements.txt`` iles,
and may arrange them either at the project root (e.g. ``requirements.txt`` and
``test-requirements.txt``) or else in a directory (e.g.
``requirements/base.txt`` and ``requirements/test.txt``). However, there are
major issues with the use of requirements files in this way:

* There is no standardized naming convention such that tools can discover or
  use these files by name.

* ``requirements.txt`` files are *not standardized*, but instead provide
  options to ``pip``.

As a result, it is difficult to define tool behaviors based on
``requirements.txt`` files. They are not trivial to discover or identify by
name, and their contents may contain a mix of package specifiers and additional
``pip`` options.

The lack of a standard for ``requirements.txt`` contents also means they are
not portable to any alternative tools which wish to process them other than
``pip``.

Additionally, ``requirements.txt`` files require a file per dependency list.
For some use-cases, this makes the marginal cost of dependency groupings high,
relative to their benefit.
A terser declaration is beneficial to projects with a number of small groups of
dependencies.

In contrast with this, Dependency Groups are defined at a well known location
in ``pyproject.toml`` with fully standardized contents. Not only will they have
immediate utility, but they will also serve as a starting point for future
standards.

Limitations of ``extras``
-------------------------

``extras`` are additional package metadata declared in the
``[project.optional-dependencies]`` table. They provide names for lists of
package specifiers which are published as part of a package's metadata, and
which a user can request under that name, as in ``pip install 'foo[bar]'`` to
install ``foo`` with the ``bar`` extra.

Because ``extras`` are package metadata, they are not usable when a project
does not build a distribution (i.e., is not a package).

For projects which are packages, ``extras`` are a common solution for defining
development dependencies, but even under these circumstances they have
downsides:

* Because an ``extra`` defines optional *additional* dependencies, it is not
  possible to install an ``extra`` without installing the current package and
  its dependencies.

* Because they are user-installable, ``extras`` are part of the public interface
  for packages. Because ``extras`` are published, package developers often are
  concerned about ensuring that their development extras are not confused with
  user-facing extras.

Rationale
=========

This PEP defines the storage of requirements data in lists within a
``[dependency-groups]`` table.
This name was chosen to match the canonical name of the feature
("Dependency Groups").

This format should be as simple and learnable as possible, having a format
very similar to existing ``requirements.txt`` files for many cases. Each list
in ``[dependency-groups]`` is defined as a list of package specifiers. For
example:

.. code-block:: toml

    [dependency-groups]
    test = ["pytest>7", "coverage"]

There are a number of use cases for ``requirements.txt`` files which require
data which cannot be expressed in :pep:`508` dependency specifiers. Such
fields are not valid in Dependency Groups. Including many of the data and
fields which ``pip`` supports, such as index servers, hashes, and path
dependencies, requires new standards. This standard leaves room for new
standards and developments, but does not attempt to support all valid
``requirements.txt`` contents.

The only exception to this is the ``-r`` flag which ``requirements.txt`` files
use to include one file in another. Dependency Groups support an "include"
mechanism which is similar in meaning, allowing one dependency group to extend
another.

Dependency Groups have two additional features which are similar to
``requirements.txt`` files:

* they are not published as distinct metadata in any built distribution

* installation of a dependency group does not imply installation of a package's
  dependencies or the package itself

Use Cases
---------

The following use cases are considered important targets for this PEP. They are
defined in greater detail in the :ref:`Use Cases Appendix <use_cases>`.

* Web Applications deployed via a non-python-packaging build process
* Libraries with unpublished dev dependency groups
* Data science projects with groups of dependencies but no core package
* *Input data* to lockfile generation (Dependency Groups should generally not
  be used as a location for locked dependency data)
* Input data to an environment manager, such as tox, Nox, or Hatch
* Configurable IDE discovery of test and linter requirements
* Exposure of package dependencies for install, without the package itself

Support for Inclusion from ``project.dependencies``
---------------------------------------------------

The ``project.dependencies`` and ``project.optional-dependencies`` tables are
allowed to include Dependency Groups, requiring an update to the specification
of these tables.

The drivers for such changes are that some usages are well solved by the
addition of such support, and that failing to include support in the initial
Dependency Group PEP, but adding such support later in a subsequent PEP,
would make the support landscape significantly more difficult for tool
maintainers.

Inclusion of a Dependency Group in ``project.dependencies`` or
``project.optional-dependencies`` takes the form of a Dependency Group Include,
defined in the specification section below.

Regarding Poetry and PDM Dependency Groups
------------------------------------------

The existing Poetry and PDM tools already offer a feature which each calls
"Dependency Groups". However, absent any standard for specifying collections
of dependencies, each tool defines these in a tool-specific way, in the
relevant sections of the ``[tool]`` table.

(PDM also uses extras for some Dependency Groups, and overlaps the notion
heavily with extras.)

This PEP does not support all of the features of Poetry and PDM, which, like
``requirements.txt`` files for ``pip``, support several non-standard extensions
to common dependency specifiers.

It should be possible for such tools to use standardized Dependency Groups as
extensions of their own Dependency Group mechanisms.
However, defining a new data format which replaces the existing Poetry and PDM
solutions is a non-goal. Doing so would require standardizing several
additional features, such as path dependencies, which are supported by these
tools.

Dependency Groups are not Hidden Extras
---------------------------------------

Dependency Groups are very similar to extras which go unpublished.
However, there are three major features which distinguish them from extras
further:

* they support non-package projects

* installation of a Dependency Group does not imply installation of a package's
  dependencies (or the package itself)

* a package's requirements (and extras) may depend upon Dependency Groups

Future Compatibility & Invalid Data
-----------------------------------

Dependency Groups are intended to be extensible in future PEPs.
However, Dependency Groups should also be usable by multiple tools in a
single Python project.
With multiple tools using the same data, it is possible that one implements
a future PEP which extends Dependency Groups, while another does not.

To support users in this case, this PEP defines and recommends validation
behaviors in which tools only examine Dependency Groups which they are using.
This allows multiple tools, using different versions of Dependency Groups data,
to share a single table in ``pyproject.toml``.

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

This PEP defines a new section (table) in ``pyproject.toml`` files named
``dependency-groups``. The ``dependency-groups`` table contains an arbitrary
number of user-defined keys, each of which has, as its value, a list of
requirements (defined below). These keys must be
`valid non-normalized names <https://packaging.python.org/en/latest/specifications/name-normalization/#valid-non-normalized-names>`__,
and must be
`normalized <https://packaging.python.org/en/latest/specifications/name-normalization/#normalization>`__
before comparisons.

Tools SHOULD prefer to present the original, non-normalized name to users by
default. If duplicate names, after normalization, are encountered, tools SHOULD
emit an error.

Requirement lists under ``dependency-groups`` may contain strings, tables
("dicts" in Python), or a mix of strings and tables.

Strings in requirement lists must be valid
`Dependency Specifiers <https://packaging.python.org/en/latest/specifications/dependency-specifiers/>`__,
as defined in :pep:`508`.

Tables in requirement lists must be valid Dependency Object Specifiers,
defined below.

The ``project`` table in ``pyproject.toml`` is modified such that
``project.dependencies`` and the values of ``project.optional-dependencies``
may contain Dependency Object Specifiers.

Dependency Object Specifiers
----------------------------

Dependency Object Specifiers are tables which define zero or more dependencies.

This PEP standardizes only one type of Dependency Object Specifier, a
"Dependency Group Include". Other types may be added in future standards.

Dependency Group Include
''''''''''''''''''''''''

A Dependency Group Include includes the dependencies of another Dependency
Group in the current Dependency Group.

An include is defined as a table with exactly one key, ``"include-group"``,
whose value is a string, the name of another Dependency Group.

For example, ``{include-group = "test"}`` is an include which expands to the
contents of the ``test`` Dependency Group.

Includes are defined to be exactly equivalent to the contents of the named
Dependency Group, inserted into the current group at the location of the include.
For example, if ``foo = ["a", "b"]`` is one group, and
``bar = ["c", {include-group = "foo"}, "d"]`` is another, then ``bar`` should
evaluate to ``["c", "a", "b", "d"]`` when Dependency Group Includes are expanded.

Dependency Group Includes may specify the same package multiple times. Tools
SHOULD NOT deduplicate or otherwise alter the list contents produced by the
include. For example, given the following table:

.. code:: toml

    [dependency-groups]
    group-a = ["foo"]
    group-b = ["foo>1.0"]
    group-c = ["foo<1.0"]
    all = ["foo", {include-group = "group-a"}, {include-group = "group-b"}, {include-group = "group-c"}]

The resolved value of ``all`` SHOULD be ``["foo", "foo", "foo>1.0", "foo<1.0"]``.
Tools should handle such a list exactly as they would handle any other case in
which they are asked to process the same requirement multiple times with
different version constraints.

Dependency Group Includes may include lists containing Dependency Group
Includes, in which case those includes should be expanded as well. Dependency
Group Includes MUST NOT include cycles, and tools SHOULD report an error if
they detect a cycle.

``project`` Table Changes
-------------------------

The ``[project]`` table, originally defined in :pep:`621` is extended in two ways.

``dependencies``

In addition to :pep:`508` strings, the array may contain Dependency Object
Specifiers.

``optional-dependencies``

In addition to :pep:`508` strings, the array values in this table may contain
Dependency Object Specifiers.

Example Dependency Groups Table
-------------------------------

The following is an example of a partial ``pyproject.toml`` which uses this to
define four Dependency Groups: ``test``, ``docs``, ``typing``, and
``typing-test``:

.. code:: toml

    [dependency-groups]
    test = ["pytest", "coverage"]
    docs = ["sphinx", "sphinx-rtd-theme"]
    typing = ["mypy", "types-requests"]
    typing-test = [{include-group = "typing"}, {include-group = "test"}, "useful-types"]

Note that none of these Dependency Group declarations implicitly install the
current package, its dependencies, or any optional dependencies.
Use of a Dependency Group like ``test`` to test a package requires that the
user's configuration or toolchain also installs ``.``. For example,

.. code-block:: shell

    $TOOL install-dependency-group test
    pip install -e .

could be used (supposing ``$TOOL`` is a tool which supports installing
Dependency Groups) to build a testing environment.

This also allows for the ``docs`` dependency group to be used without
installing the project as a package:

.. code-block:: shell

    $TOOL install-dependency-group docs

Package Building
----------------

Build backends MUST NOT include Dependency Group data in built distributions as
package metadata. This means that PKG-INFO in sdists and METADATA in wheels
do not include any referencable fields containing Dependency Groups.

It is valid to use Dependency Groups in the evaluation of dynamic metadata, and
``pyproject.toml`` files included in sdists will naturally still contain the
``[dependency-groups]`` table. However, the table contents are not part of a
published package's interfaces.

Installing Dependency Groups
----------------------------

Tools which support Dependency Groups are expected to provide new options and
interfaces to allow users to install from Dependency Groups.

No syntax is defined for expressing the Dependency Group of a package, for two
reasons:

* it would not be valid to refer to the Dependency Groups of a third-party
  package from PyPI (because the data is defined to be unpublished)

* there is not guaranteed to be a current package for Dependency Groups -- part
  of their purpose is to support non-package projects

For example, a possible pip interface for installing Dependency Groups
would be:

.. code:: shell

    pip install --dependency-groups=test,typing

Note that this is only an example. This PEP does not declare any requirements
for how tools support the installation of Dependency Groups.

Overlapping Install UX with Extras
''''''''''''''''''''''''''''''''''

Tools MAY choose to provide the same interfaces for installing Dependency
Groups as they do for installing extras.

Note that this specification does not forbid having an extra whose name matches
a Dependency Group. In such cases, tools must define their own semantics for
precedence order or disambiguation.

Users are advised to avoid creating Dependency Groups whose names match extras.
Tools SHOULD NOT treat such matching as an error.

Validation and Compatibility
----------------------------

Tools supporting Dependency Groups may want to validate data before using it.
However, tools implementing such validation behavior should be careful to allow
for future expansions to this spec, so that they do not unnecessarily emit
errors or warnings in the presence of new syntax.

Tools SHOULD error when evaluating or processing unrecognized data in
Dependency Groups.

Tools SHOULD NOT eagerly validate the list contents of **all** Dependency
Groups.

This means that in the presence of the following data, most tools will allow
the ``foo`` group to be used, and will only error when the ``bar`` group is
used:

.. code-block:: toml

    [dependency-groups]
    foo = ["pyparsing"]
    bar = [{set-phasers-to = "stun"}]

Linters and Validators may be stricter
''''''''''''''''''''''''''''''''''''''

Eager validation is discouraged for tools which primarily install or resolve
Dependency Groups.
Linters and validation tools may have good cause to ignore this recommendation.

Reference Implementation
========================

The following Reference Implementation prints the contents of a Dependency
Group to stdout, newline delimited.
The output is therefore valid ``requirements.txt`` data.

.. code-block:: python

    import re
    import sys
    import tomllib
    from collections import defaultdict

    from packaging.requirements import Requirement


    def _normalize_name(name: str) -> str:
        return re.sub(r"[-_.]+", "-", name).lower()


    def _normalize_group_names(dependency_groups: dict) -> dict:
        original_names = defaultdict(list)
        normalized_groups = {}

        for group_name, value in dependency_groups.items():
            normed_group_name = _normalize_name(group_name)
            original_names[normed_group_name].append(group_name)
            normalized_groups[normed_group_name] = value

        errors = []
        for normed_name, names in original_names.items():
            if len(names) > 1:
                errors.append(f"{normed_name} ({', '.join(names)})")
        if errors:
            raise ValueError(f"Duplicate dependency group names: {', '.join(errors)}")

        return normalized_groups


    def _resolve_dependency_group(
        dependency_groups: dict, group: str, past_groups: tuple[str] = ()
    ) -> list[str]:
        if group in past_groups:
            raise ValueError(f"Cyclic dependency group include: {group} -> {past_groups}")

        if group not in dependency_groups:
            raise LookupError(f"Dependency group '{group}' not found")

        raw_group = dependency_groups[group]
        if not isinstance(raw_group, list):
            raise ValueError(f"Dependency group '{group}' is not a list")

        realized_group = []
        for item in raw_group:
            if isinstance(item, str):
                # packaging.requirements.Requirement parsing ensures that this is a valid
                # PEP 508 Dependency Specifier
                # raises InvalidRequirement on failure
                Requirement(item)
                realized_group.append(item)
            elif isinstance(item, dict):
                if tuple(item.keys()) != ("include-group",):
                    raise ValueError(f"Invalid dependency group item: {item}")

                include_group = _normalize_name(next(iter(item.values())))
                realized_group.extend(
                    _resolve_dependency_group(
                        dependency_groups, include_group, past_groups + (group,)
                    )
                )
            else:
                raise ValueError(f"Invalid dependency group item: {item}")

        return realized_group


    def resolve(dependency_groups: dict, group: str) -> list[str]:
        if not isinstance(dependency_groups, dict):
            raise TypeError("Dependency Groups table is not a dict")
        if not isinstance(group, str):
            raise TypeError("Dependency group name is not a str")
        return _resolve_dependency_group(dependency_groups, group)


    if __name__ == "__main__":
        with open("pyproject.toml", "rb") as fp:
            pyproject = tomllib.load(fp)

        dependency_groups_raw = pyproject["dependency-groups"]
        dependency_groups = _normalize_group_names(dependency_groups_raw)
        print("\n".join(resolve(pyproject["dependency-groups"], sys.argv[1])))

Backwards Compatibility
=======================

At time of writing, the ``dependency-groups`` namespace within a
``pyproject.toml`` file is unused. Since the top-level namespace is
reserved for use only by standards specified at packaging.python.org,
there are no direct backwards compatibility concerns.

However, the introduction of the feature as a potential component of
``project`` data has implications for a number of ecosystem tools.

Audit and Update Tools
----------------------

A wide range of tools understand Python dependency data as expressed in
``project.dependencies`` and ``project.optional-dependencies`` and may
additionally support ``setup.cfg``, ``requirements.txt``, and even
``setup.py``. (e.g., Dependabot, Tidelift, etc)

Such tools inspect dependency data and, in some cases, offer tool-assisted or
fully automated updates.
It is our expectation that no such tools would support the new Dependency
Groups at first, and broad ecosystem support could take many months or even some
number of years to arrive.

As a result, users of Dependency Groups would experience a degradation in their
workflows and tool support at the time that they start using Dependency Groups.
This is true of any new standard for where and how dependency data are encoded.

Repackaging
-----------

Repackaging, and in particular tool-assisted repackaging such as Grayskull or
PyInstaller, will need to contend with the change to package metadata
definitions as well.

Because repackagers for alternate ecosystems such as conda and linux distros
are often distinct persons vs the package publishers, this compatibility
concern is more difficult to address than cases in which the package
maintainers experience the impact. Package maintainers may be unaware of the
impact of beginning to use Dependency Groups, and may unknowingly make changes
which harm downstream repackaging workflows.

There are two primary ways in which this issue can be addressed:

* via education and "How to Teach This" -- users whose packages are repackaged
  should be made aware that using new standards may cause issues for downstream
  package consumers

* by driving more behavior through build-backends -- so long as dependency
  metadata are gathered via :pep:`517` interfaces, downstream repackagers can
  remain ignorant of which build system is being used and whether or not it
  supports Dependency Groups

Security Implications
=====================

This PEP introduces new syntaxes and data formats for specifying dependency
information in projects. However, it does not introduce newly specified
mechanisms for handling or resolving dependencies.

It therefore does not carry security concerns other than those inherent in any
tools which may already be used to install dependencies -- i.e. malicious
dependencies may be specified here, just as they may be specified in
``requirements.txt`` files.

How to Teach This
=================

This feature should be referred to by its canonical name, "Dependency Groups".

The basic form of usage should be taught as a variant on typical
``requirements.txt`` data. Standard dependency specifiers (:pep:`508`) can be
added to a named list. Rather than asking pip to install from a
``requirements.txt`` file, either pip or a relevant workflow tool will install
from a named Dependency Group.

For new Python users, they may be taught directly to create a section in
``pyproject.toml`` containing their Dependency Groups, similarly to how they
are currently taught to use ``requirements.txt`` files.
This also allows new Python users to learn about ``pyproject.toml`` files
without needing to learn about package building.
A ``pyproject.toml`` file with only ``[dependency-groups]`` and no other tables
is valid.

For both new and experienced users, the Dependency Group Includes will need to
be explained. For users with experience using ``requirements.txt``, this can be
described as an analogue for ``-r``. For new users, they should be taught that
an include allows one Dependency Group to extend another. Similar configuration
interfaces and the Python ``list.extend`` method may be used to explain the
idea by analogy.

Python users who have used ``setup.py`` packaging may be familiar with common
practices which predate ``pyproject.toml``, in which package metadata is
defined dynamically. Requirements loaded from ``requirements.txt`` files and
definitions of static lists prior to ``setup()`` invocation readily analogize
with Dependency Groups.

Notes for Packages which are Repackaged
---------------------------------------

A special note should be given to package maintainers whose packages are
repackaged by linux distros, homebrew, conda, etc.

Use of Dependency Group Includes in core project metadata,
``project.dependencies`` and ``project.optional-dependencies``, may break these
consumers' uses of your package. Because they may be consuming and directly
interact with the source for your repository, their toolchains may not support
Dependency Groups at the same time that the package maintainers' tools are
updated.

Ensure that repackaging consumers can contact you if there is an issue, and
make sure to note transitions to use Dependency Groups in your changelogs.

Interfaces for Use of Dependency Groups
---------------------------------------

This specificaion provides no universal interface for interacting with
Dependency Groups, other than inclusion in a built package via the ``project``
table. This has implications both for tool authors and for users.

Tool authors should determine how or if Dependency Groups are relevant to their
user stories, and build their own interfaces to fit.
For environment managers, resolvers, installers, and related non-build tools,
they will be able to document that they support "PEP 735 Dependency Groups",
but they will be responsible for documenting their usage modes.
For build backends, supporting Dependency Groups will require support for
inclusion from the ``project`` table, but set no other strict requirements.

For users, the primary consequence is that they must consult relevant tool
documentation whenever they wish to use Dependency Groups outside of package
builds.
Users should be advised by tools, either through documentation or runtime
warnings or errors, about usages which are disrecommended or not supported.
For example, if a tool wishes to require that all Dependency Groups are
mutually compatible, containing no contradictory package specifiers, it
should document that restriction and advise users on how to appropriately
leverage Dependency Groups for its purposes.

Rejected Ideas
==============

Why not define each Dependency Group as a table?
------------------------------------------------

If our goal is to allow for future expansion, then defining each Dependency
Group as a subtable, thus enabling us to attach future keys to each group,
allows for the greatest future flexibility.

However, it also makes the structure nested more deeply, and therefore harder
to teach and learn. One of the goals of this PEP is to be an easy replacement
for many ``requirements.txt`` use-cases.

Why not define a special string syntax to extend Dependency Specifiers?
-----------------------------------------------------------------------

Earlier drafts of this specification defined syntactic forms for Dependency
Group Includes and Path Dependencies.

However, there were three major issues with this approach:

* it complicates the string syntax which must be taught, beyond PEP 508

* the resulting strings would always need to be disambiguated from PEP 508
  specifiers, complicating implementations

Why not allow for more non-PEP 508 dependency specifiers?
---------------------------------------------------------

Several use cases surfaced during discussion which need more expressive
specifiers than are possible with :pep:`508`.

"Path Dependencies", referring to local paths, and references to
``[project.dependencies]`` were of particular interest.

However, there are no existing standards for these features (excepting the
de-facto standard of ``pip``'s implementation details).

As a result, attempting to include these features in this PEP results in a
significant growth in scope, to attempt to standardize these various features
and ``pip`` behaviors.

Special attention was devoted to attempting to standardize the expression of
editable installations, as expressed by ``pip install -e`` and :pep:`660`.
However, although the creation of editable installs is standardized for build
backends, the behavior of editables is not standardized for installers.
Inclusion of editables in this PEP requires that any supporting tool allows for
the installation of editables.

Therefore, although Poetry and PDM provide syntaxes for some of these features,
they are considered insufficiently standardized at present for inclusion in
Dependency Groups.

Why is the table not named ``[run]``, ``[project.dependency-groups]``, ...?
---------------------------------------------------------------------------

There are many possible names for this concept.
It will have to live alongside the already existing ``[project.dependencies]``
and ``[project.optional-dependencies]`` tables, and possibly a new
``[external]`` dependency table as well (at time of writing, :pep:`725`, which
defines the ``[external]`` table, is in progress).

``[run]`` was a leading proposal in earlier discussions, but its proposed usage
centered around a single set of runtime dependencies. This PEP explicitly
outlines multiple groups of dependencies, which makes ``[run]`` a less
appropriate fit -- this is not just dependency data for a specific runtime
context, but for multiple contexts.

``[project.dependency-groups]`` would offer a nice parallel with
``[project.dependencies]`` and ``[project.optional-dependencies]``, but has
major downsides for non-package projects.
``[project]`` requires several keys to be defined, such as ``name`` and
``version``. Using this name would either require redefining the ``[project]``
table to allow for these keys to be absent, or else would impose a requirement
on non-package projects to define and use these keys. By extension, it would
effectively require any non-package project allow itself to be treated as a
package.

Why is pip's planned implementation of ``--only-deps`` not sufficient?
----------------------------------------------------------------------

pip currently has a feature on the roadmap to add an
`--only-deps flag <https://github.com/pypa/pip/issues/11440>`_.
This flag is intended to allow users to install package dependencies and extras
without installing the current package.

It does not address the needs of non-package projects, nor does it allow for
the installation of an extra without the package dependencies.

Why isn't <environment manager> a solution?
-------------------------------------------

Existing environment managers like tox, Nox, and Hatch already have
the ability to list inlined dependencies as part of their configuration data.
This meets many development dependency needs, and clearly associates dependency
groups with relevant tasks which can be run.
These mechanisms are *good* but they are not *sufficient*.

First, they do not address the needs of non-package projects.

Second, there is no standard for other tools to use to access these data. This
has impacts on high-level tools like IDEs and Dependabot, which cannot support
deep integration with these Dependency Groups. (For example, at time of writing
Dependabot will not flag dependencies which are pinned in ``tox.ini`` files.)

Why not support Dependency Group Includes in ``[build-system.requires]``?
-------------------------------------------------------------------------

Although it may be interesting to allow this in the future, incorporating the
proposal into the build-system table reduces the ability of users to rely on
this to bootstrap support. :pep:`517` frontends would be required to support
Dependency Groups in order to achieve this, and the versions of these frontends
are not easily controlled by packages.

By only defining build backend support, it is possible for packages to start
leveraging the new syntax and capabilities without being concerned about
controlling the environment in which the package is built and installed.

.. _prior_art:

Appendix A: Prior Art in Non-Python Languages
=============================================

This section is primarily informational and serves to document how other
language ecosystems solve similar problems.

.. _javascript_prior_art:

JavaScript and ``package.json``
-------------------------------

In the JavaScript community, packages contain a canonical configuration and
data file, similar in scope to ``pyproject.toml``, at ``package.json``.

Two keys in ``package.json`` control dependency data: ``"dependencies"`` and
``"devDependencies"``. The role of ``"dependencies"`` is effectively the same
as that of ``[project.dependencies]`` in ``pyproject.toml``, declaring the
direct dependencies of a package.

``"dependencies"`` data
'''''''''''''''''''''''

Dependency data is declared in ``package.json`` as a mapping from package names
to version specifiers.

Version specifiers support a small grammar of possible versions, ranges, and
other values, similar to Python's :pep:`440` version specifiers.

For example, here is a partial ``package.json`` file declaring a few
dependencies:

.. code-block:: json

    {
        "dependencies": {
            "@angular/compiler": "^17.0.2",
            "camelcase": "8.0.0",
            "diff": ">=5.1.0 <6.0.0"
        }
    }

The use of the ``@`` symbol is a `scope
<https://docs.npmjs.com/cli/v10/using-npm/scope>`__ which declares the package
owner, for organizationally owned packages.
``"@angular/compiler"`` therefore declares a package named ``compiler`` grouped
under ``angular`` ownership.

Dependencies Referencing URLs and Local Paths
'''''''''''''''''''''''''''''''''''''''''''''

Dependency specifiers support a syntax for URLs and Git repositories, similar
to the provisions in Python packaging.

URLs may be used in lieu of version numbers.
When used, they implicitly refer to tarballs of package source code.

Git repositories may be similarly used, including support for committish
specifiers.

Unlike :pep:`440`, NPM allows for the use of local paths to package source code
directories for dependencies. When these data are added to ``package.json`` via
the standard ``npm install --save`` command, the path is normalized to a
relative path, from the directory containing ``package.json``, and prefixed
with ``file:``. For example, the following partial ``package.json`` contains a
reference to a sibling of the current directory:

.. code-block:: json

    {
        "dependencies": {
            "my-package": "file:../foo"
        }
    }

The `official NPM documentation
<https://docs.npmjs.com/cli/v8/configuring-npm/package-json#local-paths>`__
states that local path dependencies "should not" be published to public package
repositories, but makes no statement about the inherent validity or invalidity
of such dependency data in published packages.

``"devDependencies"`` data
''''''''''''''''''''''''''

``package.json`` is permitted to contain a second section named
``"devDependencies"``, in the same format as ``"dependencies"``.
The dependencies declared in ``"devDependencies"`` are not installed by default
when a package is installed from the package repository (e.g. as part of a
dependency being resolved) but are installed when ``npm install`` is run in the
source tree containing ``package.json``.

Just as ``"dependencies"`` supports URLs and local paths, so does
``"devDependencies"``.

``"peerDependencies"`` and ``"optionalDependencies"``
'''''''''''''''''''''''''''''''''''''''''''''''''''''

There are two additional, related sections in ``package.json`` which have
relevance.

``"peerDependencies"`` declares a list of dependencies in the same format as
``"dependencies"``, but with the meaning that these are a compatibility
declaration.
For example, the following data declares compatibility with package ``foo``
version 2:

.. code-block:: json

    {
        "peerDependencies": {
            "foo": "2.x"
        }
    }

``"optionalDependencies"`` declares a list of dependencies which should be
installed if possible, but which should not be treated as failures if they are
unavailable. It also uses the same mapping format as ``"dependencies"``.

``"peerDependenciesMeta"``
~~~~~~~~~~~~~~~~~~~~~~~~~~

``"peerDependenciesMeta"`` is a section which allows for additional control
over how ``"peerDependencies"`` are treated.

Warnings about missing dependencies can be disabled by setting packages to
``optional`` in this section, as in the following sample:

.. code-block:: json

    {
        "peerDependencies": {
            "foo": "2.x"
        },
        "peerDependenciesMeta": {
            "foo": {
                "optional": true
            }
        }
    }

``--omit`` and ``--include``
''''''''''''''''''''''''''''

The ``npm install`` command supports two options, ``--omit`` and ``--include``,
which can control whether "prod", "dev", "optional", or "peer" dependencies are installed.

The "prod" name refers to dependencies listed under ``"dependencies"``.

By default, all four groups are installed when ``npm install`` is executed
against a source tree, but these options can be used to control installation
behavior more precisely.
Furthermore, these values can be declared in ``.npmrc`` files, allowing
per-user and per-project configurations to control installation behaviors.

.. _ruby_prior_art:

Ruby & Ruby Gems
----------------

Ruby projects may or may not be intended to produce packages ("gems") in the
Ruby ecosystem. In fact, the expectation is that most users of the language do
not want to produce gems and have no interest in producing their own packages.
Many tutorials do not touch on how to produce packages, and the toolchain never
requires user code to be packaged for supported use-cases.

Ruby splits requirement specification into two separate files.

- ``Gemfile``: a dedicated file which only supports requirement data in the form
  of dependency groups
- ``<package>.gemspec``: a dedicated file for declaring package (gem) metadata

The ``bundler`` tool, providing the ``bundle`` command, is the primary interface
for using ``Gemfile`` data.

The ``gem`` tool is responsible for building gems from ``.gemspec`` data, via the
``gem build`` command.

Gemfiles & bundle
'''''''''''''''''

A `Gemfile <https://bundler.io/v1.12/man/gemfile.5.html>`__ is a Ruby file
containing ``gem`` directives enclosed in any number of ``group`` declarations.
``gem`` directives may also be used outside of the ``group`` declaration, in which
case they form an implicitly unnamed group of dependencies.

For example, the following ``Gemfile`` lists ``rails`` as a project dependency.
All other dependencies are listed under groups:

.. code-block:: ruby

    source 'https://rubygems.org'

    gem 'rails'

    group :test do
      gem 'rspec'
    end

    group :lint do
      gem 'rubocop'
    end

    group :docs do
      gem 'kramdown'
      gem 'nokogiri'
    end

If a user executes ``bundle install`` with these data, all groups are
installed. Users can deselect groups by creating or modifying a bundler config
in ``.bundle/config``, either manually or via the CLI. For example, ``bundle
config set --local without 'lint:docs'``.

It is not possible, with the above data, to exclude the top-level use of the
``'rails'`` gem or to refer to that implicit grouping by name.

gemspec and packaged dependency data
''''''''''''''''''''''''''''''''''''

A `gemspec file <https://guides.rubygems.org/specification-reference/>`__ is a
ruby file containing a `Gem::Specification
<https://ruby-doc.org/stdlib-3.0.1/libdoc/rubygems/rdoc/Gem/Specification.html>`__
instance declaration.

Only two fields in a ``Gem::Specification`` pertain to package dependency data.
These are ``add_development_dependency`` and ``add_runtime_dependency``.
A ``Gem::Specification`` object also provides methods for adding dependencies
dynamically, including ``add_dependency`` (which adds a runtime dependency).

Here is a variant of the ``rails.gemspec`` file, with many fields removed or
shortened to simplify:

.. code-block:: ruby

    version = '7.1.2'

    Gem::Specification.new do |s|
      s.platform    = Gem::Platform::RUBY
      s.name        = "rails"
      s.version     = version
      s.summary     = "Full-stack web application framework."

      s.license = "MIT"
      s.author   = "David Heinemeier Hansson"

      s.files = ["README.md", "MIT-LICENSE"]

      # shortened from the real 'rails' project
      s.add_dependency "activesupport", version
      s.add_dependency "activerecord",  version
      s.add_dependency "actionmailer",  version
      s.add_dependency "activestorage", version
      s.add_dependency "railties",      version
    end

Note that there is no use of ``add_development_dependency``.
Some other mainstream, major packages (e.g. ``rubocop``) do not use development
dependencies in their gems.

Other projects *do* use this feature. For example, ``kramdown`` makes use of
development dependencies, containing the following specification in its
``Rakefile``:

.. code-block:: ruby

    s.add_dependency "rexml"
    s.add_development_dependency 'minitest', '~> 5.0'
    s.add_development_dependency 'rouge', '~> 3.0', '>= 3.26.0'
    s.add_development_dependency 'stringex', '~> 1.5.1'

The purpose of development dependencies is only to declare an implicit group,
as part of the ``.gemspec``, which can then be used by ``bundler``.

For full details, see the ``gemspec`` directive in ``bundler``\'s
`documentation on Gemfiles
<https://bundler.io/v1.12/man/gemfile.5.html#GEMSPEC-gemspec->`__.
However, the integration between ``.gemspec`` development dependencies and
``Gemfile``/``bundle`` usage is best understood via an example.

gemspec development dependency example
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Consider the following simple project in the form of a ``Gemfile`` and ``.gemspec``.
The ``cool-gem.gemspec`` file:

.. code-block:: ruby

    Gem::Specification.new do |s|
      s.author = 'Stephen Rosen'
      s.name = 'cool-gem'
      s.version = '0.0.1'
      s.summary = 'A very cool gem that does cool stuff'
      s.license = 'MIT'

      s.files = []

      s.add_dependency 'rails'
      s.add_development_dependency 'kramdown'
    end

and the ``Gemfile``:

.. code-block:: ruby

    source 'https://rubygems.org'

    gemspec

The ``gemspec`` directive in ``Gemfile`` declares a dependency on the local
package, ``cool-gem``, defined in the locally available ``cool-gem.gemspec``
file. It *also* implicitly adds all development dependencies to a dependency
group named ``development``.

Therefore, in this case, the ``gemspec`` directive is equivalent to the
following ``Gemfile`` content:

.. code-block:: ruby

    gem 'cool-gem', :path => '.'

    group :development do
      gem 'kramdown'
    end

.. _python_prior_art:

Appendix B: Prior Art in Python
===============================

In the absence of any prior standard for Dependency Groups, two known workflow
tools, PDM and Poetry, have defined their own solutions.

This section will primarily focus on these two tools as cases of prior art
regarding the definition and use of Dependency Groups in Python.

Projects are Packages
---------------------

Both PDM and Poetry treat the projects they support as packages.
This allows them to use and interact with standard ``pyproject.toml`` metadata
for some of their needs, and allows them to support installation of the
"current project" by doing a build and install using their build backends.

Effectively, this means that neither Poetry nor PDM supports non-package projects.

Non-Standard Dependency Specifiers
----------------------------------

PDM and Poetry extend :pep:`508` dependency specifiers with additional features
which are not part of any shared standard.
The two tools use slightly different approaches to these problems, however.

PDM supports specifying local paths, and editable installs, via a syntax which
looks like a set of arguments to ``pip install``. For example, the following
dependency group includes a local package in editable mode:

.. code-block:: toml

    [tool.pdm.dev-dependencies]
    mygroup = ["-e file:///${PROJECT_ROOT}/foo"]

This declares a dependency group ``mygroup`` which includes a local editable
install from the ``foo`` directory.

Poetry describes dependency groups as tables, mapping package names to
specifiers. For example, the same configuration as the above ``mygroup``
example might appear as follows under Poetry:

.. code-block:: toml

    [tool.poetry.group.mygroup]
    foo = { path = "foo", editable = true }

PDM restricts itself to a string syntax, and Poetry introduces tables which
describe dependencies.

Installing and Referring to Dependency Groups
---------------------------------------------

Both PDM and Poetry have tool-specific support for installing dependency
groups. Because both projects support their own lockfile formats, they also
both have the capability to transparently use a dependency group name to refer
to the *locked* dependency data for that group.

However, neither tool's dependency groups can be referenced natively from other
tools like ``tox``, ``nox``, or ``pip``.
Attempting to install a dependency group under ``tox``, for example, requires
an explicit call to PDM or Poetry to parse their dependency data and do the
relevant installation step.

.. _use_cases:

Appendix C: Use Cases
=====================

Web Applications
----------------

A web application (e.g. a Django or Flask app) often does not need to build a
distribution, but bundles and ships its source to a deployment toolchain.

For example, a source code repository may define Python packaging metadata as
well as containerization or other build pipeline metadata (``Dockerfile``,
etc).
The Python application is built by copying the entire repository into a
build context, installing dependencies, and bundling the result as a machine
image or container.

Such applications have dependency groups for the build, but also for linting,
testing, etc. In practice, today, these applications often define themselves as
packages to be able to use packaging tools and mechanisms like ``extras`` to
manage their dependency groups. However, they are not conceptually packages,
meant for distribution in sdist or wheel format.

Dependency Groups allow these applications to define their various dependencies
without relying on packaging metadata, and without trying to express their
needs in packaging terms.

Libraries
---------

Libraries are Python packages which build distributions (sdist and wheel) and
publish them to PyPI.

For libraries, Dependency Groups represent an alternative to ``extras`` for
defining groups of development dependencies, with the important advantages
noted above.

A library may define groups for ``test`` and ``typing`` which allow testing and
type-checking, and therefore rely on the library's own dependencies (as
specified in ``[project.dependencies]``).

Other development needs may not require installation of the package at all. For
example, a ``lint`` Dependency Group may be valid and faster to install without
the library, as it only installs tools like ``black``, ``ruff``, or ``flake8``.

``lint`` and ``test`` environments may also be valuable locations to hook in
IDE or editor support. See the case below for a fuller description of such
usage.

Here's an example Dependency Groups table which might be suitable for a
library:

.. code-block:: toml

    [dependency-groups]
    test = ["pytest<8", "coverage"]
    typing = ["mypy==1.7.1", "types-requests"]
    lint = ["black", "flake8"]
    typing-test = [{include-group = "typing"}, "pytest<8"]

Note that none of these implicitly install the library itself.
It is therefore the responsibility of any environment management toolchain to
install the appropriate Dependency Groups along with the library when needed,
as in the case of ``test``.

Data Science Projects
---------------------

Data Science Projects typically take the form of a logical collection of
scripts and utilities for processing and analyzing data, using a common
toolchain. Components may be defined in the Jupyter Notebook format (ipynb),
but rely on the same common core set of utilities.

In such a project, there is no package to build or install. Therefore,
``pyproject.toml`` currently does not offer any solution for dependency
management or declaration.

It is valuable for such a project to be able to define at least one major
grouping of dependencies. For example:

.. code-block:: toml

    [dependency-groups]
    main = ["numpy", "pandas", "matplotlib"]

However, it may also be necessary for various scripts to have additional
supporting tools. Projects may even have conflicting or incompatible tools or
tool versions for different components, as they evolve over time.

Consider the following more elaborate configuration:

.. code-block:: toml

    [dependency-groups]
    main = ["numpy", "pandas", "matplotlib"]
    scikit = [{include-group = "main"}, "scikit-learn==1.3.2"]
    scikit-old = [{include-group = "main"}, "scikit-learn==0.24.2"]

This defines ``scikit`` and ``scikit-old`` as two similar variants of the
common suite of dependencies, pulling in different versions of ``scikit-learn``
to suit different scripts.

This PEP only defines these data. It does not formalize any mechanism for a
Data Science Project (or any other type of project) to install the dependencies
into known environments or associate those environments with the various
scripts. Such combinations of data are left as a problem for tool authors to
solve, and perhaps eventually standardize.

Lockfile Generation
-------------------

There are a number of tools which generate lockfiles in the Python ecosystem
today. PDM and Poetry each use their own lockfile formats, and pip-tools
generates ``requirements.txt`` files with version pins and hashes.

Dependency Groups are not an appropriate place to store lockfiles, as they lack
many of the necessary features. Most notably, they cannot store hashes, which
most lockfile users consider essential.

However, Dependency Groups are a valid input to tools which generate lockfiles.
Furthermore, PDM and Poetry both allow a Dependency Group name (under their
notions of Dependency Groups) to be used to refer to its locked variant.

Therefore, consider a tool which produces lockfiles, here called ``$TOOL``.
It might be used as follows:

.. code:: shell

    $TOOL lock --dependency-group=test
    $TOOL install --dependency-group=test --use-locked

All that such a tool needs to do is to ensure that its lockfile data records
the name ``test`` in order to support such usage.

The mutual compatibility of Dependency Groups is not guaranteed. For example,
the Data Science example above shows conflicting versions of ``scikit-learn``.
Therefore, installing multiple locked dependency groups in tandem may require
that tools apply additional constraints or generate additional lockfile data.
These problems are considered out of scope for this PEP.

As two examples of how combinations might be locked:

* A tool might require that lockfile data be explicitly generated for any
  combination to be considered valid

* Poetry implements the requirement that all Dependency Groups be mutually
  compatible, and generates only one locked version. (Meaning it finds a single
  solution, rather than a set or matrix of solutions.)

Environment Manager Inputs
--------------------------

A common usage in tox, Nox, and Hatch is to install a set of dependencies into
a testing environment.

For example, under ``tox.ini``, type checking dependencies may be defined
inline:

.. code-block:: ini

    [testenv:typing]
    deps =
        pyright
        useful-types
    commands = pyright src/

This combination provides a desirable developer experience within a limited
context. Under the relevant environment manager, the dependencies which are
needed for the test environment are declared alongside the commands which need
those dependencies. They are not published in package metadata, as ``extras``
would be, and they are discoverable for the tool which needs them to build the
relevant environment.

Dependency Groups apply to such usages by effectively "lifting" these
requirements data from a tool-specific location into a more broadly available
one. In the example above, only ``tox`` has access to the declared list of
dependencies. Under an implementation supporting dependency groups, the same
data might be available in a Dependency Group:

.. code-block:: toml

    [dependency-groups]
    typing = ["pyright", "useful-types"]

The data can then be used under multiple tools. For example, ``tox`` might
implement support as ``dependency_groups = typing``, replacing the ``deps``
usage above.

In order for Dependency Groups to be a viable alternative for users of
environment managers, the environment managers will need to support processing
Dependency Groups similarly to how they support inline dependency declaration.

IDE and Editor Use of Requirements Data
---------------------------------------

IDE and editor integrations may benefit from conventional or configurable name
definitions for Dependency Groups which are used for integrations.

There are at least two known scenarios in which it is valuable for an editor or
IDE to be capable of discovering the non-published dependencies of a project:

* testing: IDEs such as VS Code support GUI interfaces for running particular
  tests

* linting: editors and IDEs often support linting and autoformatting
  integrations which highlight or autocorrect errors

These cases could be handled by defining conventional group names like
``test``, ``lint``, and ``fix``, or by defining configuration mechanisms which
allow the selection of Dependency Groups.

For example, the following ``pyproject.toml`` declares the three aforementioned
groups:

.. code-block:: toml

    [dependency-groups]
    test = ["pytest", "pytest-timeout"]
    lint = ["flake8", "mypy"]
    fix = ["black", "isort", "pyupgrade"]

This PEP makes no attempt to standardize such names or reserve them for such
uses. IDEs may standardize or may allow users to configure the group names used
for various purposes.

This declaration allows the project author's knowledge of the appropriate tools
for the project to be shared with all editors of that project.

Exposure of package dependencies without the package itself
-----------------------------------------------------------

There are a variety of use-cases in which package developers would like to
install only the dependencies of a package, without the package itself.

For example:

* Specify different environment variables or options when building dependencies
  vs when building the package itself

* Creating layered container images in which the dependencies are isolated from
  the package being installed

* Providing the dependencies to analysis environments (e.g., type checking)
  without having to build and install the package itself

For an example of the last case, consider the following sample
``pyproject.toml``:

.. code-block:: toml

    [project]
    dependencies = [{include = "runtime"}]
    [optional-dependencies]
    foo = [{include = "foo"}]
    [dependency-groups]
    runtime = ["a", "b"]
    foo = ["c", "d"]
    typing = ["mypy", {include = "runtime"}, {include = "foo"}]

In this case, a ``typing`` group can be defined, with all of the package's
runtime dependencies, but without the package itself. This allows uses of the
``typing`` Dependency Group to skip installation of the package -- not only is
this more efficient, but it may reduce the requirements for testing systems.

Copyright
=========

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