Add support for license expressions

doc/pyproject_toml.rst

@@ -96,7 +96,8 @@ requires-python
   A version specifier for the versions of Python this requires, e.g. ``~=3.3`` or
   ``>=3.3,<4``, which are equivalents.
 license
-  A table with either a ``file`` key (a relative path to a license file) or a
+  A valid SPDX `license expression <https://peps.python.org/pep-0639/#term-license-expression>`_
+  or a table with either a ``file`` key (a relative path to a license file) or a
   ``text`` key (the license text).
 authors
   A list of tables with ``name`` and ``email`` keys (both optional) describing

flit_core/flit_core/common.py

@@ -336,6 +336,7 @@ class Metadata(object):
     maintainer = None
     maintainer_email = None
     license = None
+    license_expression = None
     description = None
     keywords = None
     download_url = None
@@ -398,7 +399,6 @@ def write_metadata_file(self, fp):
         optional_fields = [
             'Summary',
             'Home-page',
-            'License',
             'Keywords',
             'Author',
             'Author-email',
@@ -422,6 +422,17 @@ def write_metadata_file(self, fp):
                 value = '\n        '.join(value.splitlines())
                 fp.write(u"{}: {}\n".format(field, value))
 
+
+        license_expr = getattr(self, self._normalise_field_name("License-Expression"))
+        license = getattr(self, self._normalise_field_name("License"))
+        if license_expr:
+            # TODO: License-Expression requires Metadata-Version '2.4'
+            # Backfill it to the 'License' field for now
+            # fp.write(u'License-Expression: {}\n'.format(license_expr))
+            fp.write(u'License: {}\n'.format(license_expr))
+        elif license:
+            fp.write(u'License: {}\n'.format(license))
+
         for clsfr in self.classifiers:
             fp.write(u'Classifier: {}\n'.format(clsfr))
 

flit_core/flit_core/config.py

@@ -445,6 +445,14 @@ def _check_type(d, field_name, cls):
             "{} field should be {}, not {}".format(field_name, cls, type(d[field_name]))
         )
 
+def _check_types(d, field_name, cls_list) -> None:
+    if not isinstance(d[field_name], cls_list):
+        raise ConfigError(
+            "{} field should be {}, not {}".format(
+                field_name, ' or '.join(map(str, cls_list)), type(d[field_name])
+            )
+        )
+
 def _check_list_of_str(d, field_name):
     if not isinstance(d[field_name], list) or not all(
         isinstance(e, str) for e in d[field_name]
@@ -526,30 +534,35 @@ def read_pep621_metadata(proj, path) -> LoadedConfig:
         md_dict['requires_python'] = proj['requires-python']
 
     if 'license' in proj:
-        _check_type(proj, 'license', dict)
-        license_tbl = proj['license']
-        unrec_keys = set(license_tbl.keys()) - {'text', 'file'}
-        if unrec_keys:
-            raise ConfigError(
-                "Unrecognised keys in [project.license]: {}".format(unrec_keys)
-            )
+        _check_types(proj, 'license', (str, dict))
+        if isinstance(proj['license'], str):
+            license_expr = proj['license']
+            # TODO Validate and normalize license expression
+            md_dict['license_expression'] = license_expr
+        else:
+            license_tbl = proj['license']
+            unrec_keys = set(license_tbl.keys()) - {'text', 'file'}
+            if unrec_keys:
+                raise ConfigError(
+                    "Unrecognised keys in [project.license]: {}".format(unrec_keys)
+                )
 
-        # TODO: Do something with license info.
-        # The 'License' field in packaging metadata is a brief description of
-        # a license, not the full text or a file path. PEP 639 will improve on
-        # how licenses are recorded.
-        if 'file' in license_tbl:
-            if 'text' in license_tbl:
+            # The 'License' field in packaging metadata is a brief description of
+            # a license, not the full text or a file path.
+            if 'file' in license_tbl:
+                if 'text' in license_tbl:
+                    raise ConfigError(
+                        "[project.license] should specify file or text, not both"
+                    )
+                lc.referenced_files.append(license_tbl['file'])
+            elif 'text' in license_tbl:
+                license = license_tbl['text']
+                # TODO Normalize license if it's a valid SPDX expression
+                md_dict['license'] = license
+            else:
                 raise ConfigError(
-                    "[project.license] should specify file or text, not both"
+                    "file or text field required in [project.license] table"
                 )
-            lc.referenced_files.append(license_tbl['file'])
-        elif 'text' in license_tbl:
-            pass
-        else:
-            raise ConfigError(
-                "file or text field required in [project.license] table"
-            )
 
     if 'authors' in proj:
         _check_type(proj, 'authors', list)
@@ -565,6 +578,16 @@ def read_pep621_metadata(proj, path) -> LoadedConfig:
 
     if 'classifiers' in proj:
         _check_list_of_str(proj, 'classifiers')
+        classifiers = proj['classifiers']
+        license_expr = md_dict.get('license_expression', None)
+        if license_expr:
+            for cl in classifiers:
+                if not cl.startswith('License :: '):
+                    continue
+                raise ConfigError(
+                    "License classifier are deprecated in favor of the license expression. "
+                    "Remove the '{}' classifier".format(cl)
+                )
         md_dict['classifiers'] = proj['classifiers']
 
     if 'urls' in proj:

flit_core/pyproject.toml

@@ -12,9 +12,8 @@ description = "Distribution-building parts of Flit. See flit package for more in
 dependencies = []
 requires-python = '>=3.6'
 readme = "README.rst"
-license = {file = "LICENSE"}
+license = "BSD-3-Clause"
 classifiers = [
-    "License :: OSI Approved :: BSD License",
     "Topic :: Software Development :: Libraries :: Python Modules",
 ]
 dynamic = ["version"]

flit_core/tests_core/test_common.py

@@ -205,3 +205,27 @@ def test_metadata_2_3_provides_extra(provides_extra, expected_result):
     msg = email.parser.Parser(policy=email.policy.compat32).parse(sio)
     assert msg['Provides-Extra'] == expected_result
     assert not msg.defects
+
+@pytest.mark.parametrize(
+    ('value', 'expected_license', 'expected_license_expression'),
+    [
+        ({'license': 'MIT'}, 'MIT', None),
+        ({'license_expression': 'MIT'}, 'MIT', None),  # TODO Metadata 2.4
+        ({'license_expression': 'Apache-2.0'}, 'Apache-2.0', None)  # TODO Metadata 2.4
+    ],
+)
+def test_metadata_license(value, expected_license, expected_license_expression):
+    d = {
+        'name': 'foo',
+        'version': '1.0',
+        **value,
+    }
+    md = Metadata(d)
+    sio = StringIO()
+    md.write_metadata_file(sio)
+    sio.seek(0)
+
+    msg = email.parser.Parser(policy=email.policy.compat32).parse(sio)
+    assert msg.get('License') == expected_license
+    assert msg.get('License-Expression') == expected_license_expression
+    assert not msg.defects

flit_core/tests_core/test_config.py

@@ -1,5 +1,7 @@
 import logging
+import re
 from pathlib import Path
+from unittest.mock import patch
 import pytest
 
 from flit_core import config
@@ -139,6 +141,12 @@ def test_bad_include_paths(path, err_match):
     ({'license': {'fromage': 2}}, '[Uu]nrecognised'),
     ({'license': {'file': 'LICENSE', 'text': 'xyz'}}, 'both'),
     ({'license': {}}, 'required'),
+    ({'license': 1}, "license field should be <class 'str'> or <class 'dict'>, not <class 'int'>"),
+    # ({'license': "MIT License"}, "Invalid license expression: 'MIT License'"),  # TODO
+    (
+        {'license': 'MIT', 'classifiers': ['License :: OSI Approved :: MIT License']},
+        "License classifier are deprecated in favor of the license expression",
+    ),
     ({'keywords': 'foo'}, 'list'),
     ({'keywords': ['foo', 7]}, 'strings'),
     ({'entry-points': {'foo': 'module1:main'}}, 'entry-point.*tables'),
@@ -178,3 +186,26 @@ def test_bad_pep621_readme(readme, err_match):
     }
     with pytest.raises(config.ConfigError, match=err_match):
         config.read_pep621_metadata(proj, samples_dir / 'pep621')
+
+@pytest.mark.parametrize(('value', 'license', 'license_expression'), [
+    # Normalize SPDX expressions but accept all strings for 'license = {text = ...}'
+    ('{text = "mit"}', "mit", None),  # TODO should be "MIT"
+    ('{text = "Apache Software License"}', "Apache Software License", None),
+    ('{text = "mit"}\nclassifiers = ["License :: OSI Approved :: MIT License"]', "mit", None),  # TODO should be "MIT"
+    # Accept and normalize valid SPDX expressions for 'license = ...'
+    ('"mit"', None, "mit"),  # TODO should be "MIT"
+    ('"apache-2.0"', None, "apache-2.0"),  # TODO should be "Apache-2.0"
+    ('"mit and (apache-2.0 or bsd-2-clause)"', None, "mit and (apache-2.0 or bsd-2-clause)"),  # TODO should be "MIT AND (Apache-2.0 OR BSD-2-Clause)"
+    ('"LicenseRef-Public-Domain"', None, "LicenseRef-Public-Domain"),
+])
+def test_pep621_license(value, license, license_expression):
+    path = samples_dir / 'pep621' / 'pyproject.toml'
+    data = path.read_text()
+    data = re.sub(
+        r"(^license = )(?:\{.*\})", r"\g<1>{}".format(value),
+        data, count=1, flags=re.M,
+)
+    with patch("pathlib.Path.read_text", return_value=data):
+        info = config.read_flit_config(path)
+        assert info.metadata.get('license', None) == license
+        assert info.metadata.get('license_expression', None) == license_expression