Add initial doc for Dependency Groups

source/specifications/dependency-groups.rst

@@ -0,0 +1,250 @@
+.. _dependency-groups:
+
+=================
+Dependency Groups
+=================
+
+This specification defines Dependency Groups, a mechanism for storing package
+requirements in ``pyproject.toml`` files such that they are not included in
+project metadata when it is built.
+
+Dependency Groups are suitable for internal development use-cases like linting
+and testing, as well as for projects which are not built for distribution, like
+collections of related scripts.
+
+Fundamentally, Dependency Groups should be thought of as being a standardized
+subset of the capabilities of ``requirements.txt`` files (which are
+``pip``-specific).
+
+Specification
+=============
+
+Examples
+--------
+
+This is a simple table which shows a ``test`` group::
+
+    [dependency-groups]
+    test = ["pytest>7", "coverage"]
+
+and a similar table which defines ``test`` and ``coverage`` groups::
+
+    [dependency-groups]
+    coverage = ["coverage[toml]"]
+    test = ["pytest>7", {include-group = "coverage"}]
+
+The ``[dependency-groups]`` Table
+---------------------------------
+
+Dependency Groups are defined as a table in ``pyproject.toml`` 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.
+
+``[dependency-groups]`` keys, sometimes also called "group names", must be
+:ref:`valid non-normalized names <name-format>`. Tools which handle Dependency
+Groups MUST :ref:`normalize <name-normalization>` these names before
+comparisons.
+
+Tools SHOULD prefer to present the original, non-normalized name to users, and
+if duplicate names are detected after normalization, tools SHOULD emit an
+error.
+
+Requirement lists, the values in ``[dependency-groups]``, may contain strings,
+tables (``dict`` in Python), or a mix of strings and tables. Strings must be
+valid :ref:`dependency specifiers <dependency-specifiers>`, and tables must be
+valid Dependency Group Includes.
+
+Dependency Group Include
+------------------------
+
+A Dependency Group Include includes another Dependency Group in the current
+group.
+
+An include is a table with exactly one key, ``"include-group"``, whose value is
+a string, the name of another 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-block:: 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 groups 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.
+
+Package Building
+----------------
+
+Build backends MUST NOT include Dependency Group data in built distributions as
+package metadata. This means that sdist ``PKG-INFO`` and wheel ``METADATA``
+files should not include referenceable fields containing Dependency Groups.
+
+It is, however, valid to use Dependency Groups in the evaluation of dynamic
+metadata, and ``pyproject.toml`` files included in sdists will still contain
+``[dependency-groups]``. However, the table's contents are not part of a built
+package's interfaces.
+
+Installing Dependency Groups & Extras
+-------------------------------------
+
+There is no syntax or specification-defined interface for installing or
+referring to Dependency Groups. Tools are expected to provide dedicated
+interfaces for this purpose.
+
+Tools MAY choose to provide the same or similar interfaces for interacting
+with Dependency Groups as they do for managing extras. Tools authors are
+advised that the specification does not forbid having an extra whose name
+matches a Dependency Group. Separately, users are advised to avoid creating
+Dependency Groups whose names match extras, and tools MAY treat such matching
+as an error.
+
+Validation and Compatibility
+----------------------------
+
+Tools supporting Dependency Groups may want to validate data before using it.
+When implementing such validation, authors should be aware of the possibility
+of future extensions to the specification, so that they do not unnecessarily
+emit errors or warnings.
+
+Tools SHOULD error when evaluating or processing unrecognized data in
+Dependency Groups.
+
+Tools SHOULD NOT eagerly validate the contents of *all* Dependency Groups
+unless they have a need to do so.
+
+This means that in the presence of the following data, most tools should allow
+the ``foo`` group to be used and only error if the ``bar`` group is used:
+
+.. code-block:: toml
+
+    [dependency-groups]
+    foo = ["pyparsing"]
+    bar = [{set-phasers-to = "stun"}]
+
+.. note::
+
+    There are several known cases of tools which have good cause to be
+    stricter. Linters and validators are an example, as their purpose is to
+    validate the contents of all Dependency Groups.
+
+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])))
+
+History
+=======
+
+- October 2024: This specification was approved through :pep:`735`.

source/specifications/section-distribution-metadata.rst

@@ -10,5 +10,6 @@ Package Distribution Metadata
    version-specifiers
    dependency-specifiers
    pyproject-toml
+   dependency-groups
    inline-script-metadata
    platform-compatibility-tags