Specify the valid SPDX license expressions (closes #1828)

Author: befeleme

source/specifications/core-metadata.rst

@@ -468,8 +468,8 @@ License-Expression
 .. versionadded:: 2.4
 
 Text string that is a valid SPDX
-`license expression <https://peps.python.org/pep-0639/#term-license-expression>`__
-as `defined in PEP 639 <https://peps.python.org/pep-0639/#spdx>`__.
+:term:`license expression <License Expression>`,
+as specified in :doc:`/specifications/license-expression`.
 
 Examples::
 

source/specifications/license-expression.rst

@@ -0,0 +1,56 @@
+==================
+License Expression
+==================
+
+:pep:`639` defined a new :ref:`pyproject.toml's license <pyproject-toml-license>`
+value and added a corresponding :ref:`core metadata License-Expression field
+<core-metadata-license-expression>`.
+This specification defines which license expressions are acceptable.
+
+
+Specification
+=============
+
+License can be defined as a text string that is a valid SPDX
+:term:`license expression <License Expression>`,
+as documented in the `SPDX specification <spdxpression_>`__,
+either Version 2.2 or a later compatible version.
+
+A license expression can use the following license identifiers:
+
+- Any SPDX-listed license short-form identifiers that are published in
+  the `SPDX License List <spdxlist_>`__,
+  version 3.17 or any later compatible version.
+
+- The custom ``LicenseRef-[idstring]`` string(s), where ``[idstring]`` is
+  a unique string containing letters, numbers, ``.`` and/or ``-``,
+  to identify licenses that are not included in the SPDX license list.
+  The custom identifiers must follow the SPDX specification,
+  `clause 10.1 <spdxcustom_>`__ of the given specification version.
+
+
+Examples of valid license expressions:
+
+.. code-block:: yaml
+
+    MIT
+    BSD-3-Clause
+    MIT AND (Apache-2.0 OR BSD-2-Clause)
+    MIT OR GPL-2.0-or-later OR (FSFUL AND BSD-2-Clause)
+    GPL-3.0-only WITH Classpath-Exception-2.0 OR BSD-3-Clause
+    LicenseRef-Special-License OR CC0-1.0 OR Unlicense
+    LicenseRef-Proprietary
+
+
+Examples of invalid license expressions:
+
+.. code-block:: yaml
+
+    Use-it-after-midnight  # No `LicenseRef` prefix
+    Apache-2.0 OR 2-BSD-Clause  # 2-BSD-Clause is not a valid SPDX identifier
+    LicenseRef-License with spaces  # spaces are not allowed
+    LicenseRef-License_with_underscores  # underscore are not allowed
+
+.. _spdxcustom: https://spdx.github.io/spdx-spec/v2.2.2/other-licensing-information-detected/
+.. _spdxlist: https://spdx.org/licenses/
+.. _spdxpression: https://spdx.github.io/spdx-spec/v2.2.2/SPDX-license-expressions/

source/specifications/pyproject-toml.rst

@@ -254,7 +254,9 @@ The Python version requirements of the project.
 - Corresponding :ref:`core metadata <core-metadata>` field:
   :ref:`License-Expression <core-metadata-license-expression>`
 
-Text string that is a valid SPDX license expression as defined in :pep:`639`.
+Text string that is a valid SPDX
+:term:`license expression <License Expression>`,
+as specified in :doc:`/specifications/license-expression`.
 Tools SHOULD validate and perform case normalization of the expression.
 
 The table subkeys of the ``license`` key are deprecated.

source/specifications/section-distribution-metadata.rst

@@ -15,3 +15,4 @@ Package Distribution Metadata
    platform-compatibility-tags
    well-known-project-urls
    glob-patterns
+   license-expression