Add support for PEP 639 change from 'License' to 'License-Expression' in importlib.metadata

[bskinn]

EDIT: Just scanned the PR list, like I should have before putting this together. This PR is an alternative to #213; it seems like this smaller fix may be sufficient? According to the metadata spec, License and License-Expression are mutually exclusive, and License-Expression is expressly declared as the replacement for License, so it should never be necessary to handle package metadata where both fields appear.

---

PEP 639 made some changes to the packaging metadata standard, including deprecating License in favor of License-Expression. This has led to pip-licenses failing to locate various packages' licenses for releases after those packages' build systems rolled over onto PEP 639-compliant tooling.

This PR adds a second lambda to the lookup for the "license" metadata key, pulling the "license-expression" metadata entry, so that metadata with either key/field can be properly processed.

Given that License-Expression is now the standard and License is deprecated, there might be semantic reasons to put the check for "license-expression" first; I figure that's your call.

Example environment:

$ pip list
Package            Version Editable project location
------------------ ------- ----------------------------
black              25.1.0
build              1.3.0
click              8.2.1
coverage           7.10.1
docutils           0.22
iniconfig          2.1.0
isort              6.0.1
mypy_extensions    1.1.0
packaging          25.0
pathspec           0.12.1
pip                25.2
pip-licenses       5.0.0   .../pip-licenses
pip-tools          7.5.0
platformdirs       4.3.8
pluggy             1.6.0
prettytable        3.16.0
pycodestyle        2.14.0
Pygments           2.19.2
pyproject_hooks    1.2.0
pytest             8.4.1
pytest-cov         6.2.1
pytest-pycodestyle 2.5.0
setuptools         80.9.0
tomli              2.2.1
tomli_w            1.2.0
wcwidth            0.2.13
wheel              0.45.1

Before the fix (my arrows added):

$ pip-licenses
 Name                Version  License
 Pygments            2.19.2   BSD License
 black               25.1.0   MIT License
 build               1.3.0    UNKNOWN          <<<<<
 click               8.2.1    UNKNOWN          <<<<<
 coverage            7.10.1   Apache-2.0
 docutils            0.22     BSD License; GNU General Public License (GPL); Public Domain
 iniconfig           2.1.0    MIT License
 isort               6.0.1    MIT License
 mypy_extensions     1.1.0    UNKNOWN          <<<<<
 packaging           25.0     Apache Software License; BSD License
 pathspec            0.12.1   Mozilla Public License 2.0 (MPL 2.0)
 pip-tools           7.5.0    BSD License
 platformdirs        4.3.8    MIT License
 pluggy              1.6.0    MIT License
 pycodestyle         2.14.0   MIT
 pyproject_hooks     1.2.0    MIT License
 pytest              8.4.1    MIT License
 pytest-cov          6.2.1    MIT
 pytest-pycodestyle  2.5.0    UNKNOWN          <<<<<
 tomli_w             1.2.0    MIT License

After the fix:

$ pip-licenses
 Name                Version  License
 Pygments            2.19.2   BSD License
 black               25.1.0   MIT License
 build               1.3.0    MIT              <<<<<
 click               8.2.1    BSD-3-Clause     <<<<<
 coverage            7.10.1   Apache-2.0
 docutils            0.22     BSD License; GNU General Public License (GPL); Public Domain
 iniconfig           2.1.0    MIT License
 isort               6.0.1    MIT License
 mypy_extensions     1.1.0    MIT              <<<<<
 packaging           25.0     Apache Software License; BSD License
 pathspec            0.12.1   Mozilla Public License 2.0 (MPL 2.0)
 pip-tools           7.5.0    BSD License
 platformdirs        4.3.8    MIT License
 pluggy              1.6.0    MIT License
 pycodestyle         2.14.0   MIT
 pyproject_hooks     1.2.0    MIT License
 pytest              8.4.1    MIT License
 pytest-cov          6.2.1    MIT
 pytest-pycodestyle  2.5.0    MIT              <<<<<
 tomli_w             1.2.0    MIT License

The test suite had some failing tests on master branch immediately after clone:

================================================================================= tests coverage ==================================================================================
________________________________________________________________ coverage: platform linux, python 3.12.11-final-0 _________________________________________________________________

Name             Stmts   Miss  Cover   Missing
----------------------------------------------
piplicenses.py     458      2    99%   211, 909
----------------------------------------------
TOTAL              458      2    99%
============================================================================= short test summary info =============================================================================
FAILED test_piplicenses.py::TestGetLicenses::test_from_all - AssertionError: 'Apache 2.0' not found in ['BSD', 'Apache-2.0', 'UNKNOWN', 'UNKNOWN', 'UNKNOWN', 'UNKNOWN', 'UNKNOWN', 'UNKNOWN', 'MIT', 'UNKNOWN', 'BSD-2-Clause', 'UNKNOWN',...
FAILED test_piplicenses.py::TestGetLicenses::test_with_normalized_packages - AssertionError: Lists differ: ['typing_extensions'] != []
FAILED test_piplicenses.py::TestGetLicenses::test_with_packages - AssertionError: Lists differ: ['py'] != []
FAILED test_piplicenses.py::test_allow_only - AssertionError: assert 'license MIT License not in allow-only licenses was found for package' in 'license MIT not in allow-only licenses was found for package pycodestyle:2.1...
FAILED test_piplicenses.py::test_allow_only_partial - AssertionError: assert 'license MIT License not in allow-only licenses was found for package' in 'license MIT not in allow-only licenses was found for package pycodestyle:2.1...
FAILED test_piplicenses.py::test_fail_on_partial_match - AssertionError: assert 'fail-on license MIT License was found for package' in 'fail-on license MIT was found for package pycodestyle:2.14.0\n'
============================================================== 6 failed, 66 passed, 1 skipped, 82 warnings in 7.34s ===============================================================

After the change of this PR, the test results and coverage are unchanged:

================================================================================= tests coverage ==================================================================================
________________________________________________________________ coverage: platform linux, python 3.12.11-final-0 _________________________________________________________________

Name             Stmts   Miss  Cover   Missing
----------------------------------------------
piplicenses.py     458      2    99%   214, 912
----------------------------------------------
TOTAL              458      2    99%
============================================================================= short test summary info =============================================================================
FAILED test_piplicenses.py::TestGetLicenses::test_from_all - AssertionError: 'Apache 2.0' not found in ['BSD', 'Apache-2.0', 'UNKNOWN', 'MIT', 'UNKNOWN', 'MIT', 'MIT', 'UNKNOWN', 'MIT', 'UNKNOWN', 'BSD-2-Clause', 'UNKNOWN', 'MIT', 'MIT...
FAILED test_piplicenses.py::TestGetLicenses::test_with_normalized_packages - AssertionError: Lists differ: ['typing_extensions'] != []
FAILED test_piplicenses.py::TestGetLicenses::test_with_packages - AssertionError: Lists differ: ['py'] != []
FAILED test_piplicenses.py::test_allow_only - AssertionError: assert 'license MIT License not in allow-only licenses was found for package' in 'license MIT not in allow-only licenses was found for package pycodestyle:2.1...
FAILED test_piplicenses.py::test_allow_only_partial - AssertionError: assert 'license MIT License not in allow-only licenses was found for package' in 'license MIT not in allow-only licenses was found for package pycodestyle:2.1...
FAILED test_piplicenses.py::test_fail_on_partial_match - AssertionError: assert 'fail-on license MIT License was found for package' in 'fail-on license MIT was found for package pycodestyle:2.14.0\n'
============================================================== 6 failed, 66 passed, 1 skipped, 82 warnings in 7.14s ===============================================================

The line numbers of the coverage misses are offset due to the lines I added in the PR.

[reactive-firewall]

Closing as Superseded by PR #238