Support simple SPDX license expressions (PEP 639)

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).
 license-files
   A list of glob patterns for license files to include.

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
@@ -399,7 +400,6 @@ def write_metadata_file(self, fp):
         optional_fields = [
             'Summary',
             'Home-page',
-            'License',
             'Keywords',
             'Author',
             'Author-email',
@@ -423,6 +423,17 @@ 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

@@ -495,6 +495,14 @@
             "{} 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]
@@ -577,30 +585,38 @@
 
     license_files = set()
     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):
+            md_dict['license_expression'] = normalize_license_expr(proj['license'])
+        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"
+                    )
+                license_f = license_tbl['file']
+                if isabs_ish(license_f):
+                    raise ConfigError(
+                        f"License file path ({license_f}) cannot be an absolute path"
+                    )
+                if not (path.parent / license_f).is_file():
+                    raise ConfigError(f"License file {license_f} does not exist")
+                license_files.add(license_tbl['file'])
+            elif 'text' in license_tbl:
+                pass
+            else:
                 raise ConfigError(
-                    "[project.license] should specify file or text, not both"
+                    "file or text field required in [project.license] table"
                 )
-            license_files.add(license_tbl['file'])
-        elif 'text' in license_tbl:
-            pass
-        else:
-            raise ConfigError(
-                "file or text field required in [project.license] table"
-            )
 
     if 'license-files' in proj:
         _check_type(proj, 'license-files', list)
@@ -635,6 +651,16 @@
 
     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:
@@ -788,3 +814,36 @@
     absolute paths, we also want to reject these odd halfway paths.
     """
     return os.path.isabs(path) or path.startswith(('/', '\\'))
+
+
+def normalize_license_expr(s: str):
+    """Validate & normalise an SPDX license expression
+
+    For now this only handles simple expressions (referring to 1 license)
+    """
+    from ._spdx_data import licenses
+    ls = s.lower()
+    if ls.startswith('licenseref-'):
+        ref = s.partition('-')[2]
+        if re.match(r'([a-zA-Z0-9\-.])+$', ref):
+            # Normalise case of LicenseRef, leave the rest alone
+            return "LicenseRef-" + ref
+        raise ConfigError(
+            "LicenseRef- license expression can only contain ASCII letters "
+            "& digits, - and ."
+        )
+
+    or_later = s.endswith('+')
+    if or_later:
+        ls = ls[:-1]
+
+    try:
+        info = licenses[ls]
+    except KeyError:
+        if os.environ.get('FLIT_ALLOW_INVALID'):
+            log.warning("Invalid license ID {!r} allowed by FLIT_ALLOW_INVALID"
+                        .format(s))
+            return s
+        raise ConfigError(f"{s!r} is not a recognised SPDX license ID")
+
+    return info['id'] + ('+' if or_later else '')

flit_core/pyproject.toml

@@ -12,9 +12,9 @@ description = "Distribution-building parts of Flit. See flit package for more in
 dependencies = []
 requires-python = '>=3.6'
 readme = "README.rst"
+license = "BSD-3-Clause"
 license-files = ["LICENSE*", "flit_core/vendor/**/LICENSE*"]
 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

@@ -140,6 +140,12 @@ 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",
+    ),
     ({'license-files': 1}, r"\blist\b"),
     ({'license-files': ["/LICENSE"]}, r"'/LICENSE'.+must not start with '/'"),
     ({'license-files': ["../LICENSE"]}, r"'../LICENSE'.+must not contain '..'"),
@@ -202,6 +208,37 @@ def test_bad_pep621_readme(readme, err_match):
         config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
 
 
+@pytest.mark.parametrize(('value', 'license_expression'), [
+    # Accept and normalize valid SPDX expressions for 'license = ...'
+    ("mit",  "MIT"),
+    ("apache-2.0", "Apache-2.0"),
+    ("APACHE-2.0+", "Apache-2.0+"),
+    # TODO: compound expressions
+    #("mit and (apache-2.0 or bsd-2-clause)", "MIT AND (Apache-2.0 OR BSD-2-Clause)"),
+    # LicenseRef expressions: only the LicenseRef is normalised
+    ("LiceNseref-Public-DoMain", "LicenseRef-Public-DoMain"),
+])
+def test_license_expr(value, license_expression):
+    proj = {
+        'name': 'module1', 'version': '1.0', 'description': 'x', 'license': value
+    }
+    info = config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
+    assert 'license' not in info.metadata
+    assert info.metadata['license_expression'] == license_expression
+
+def test_license_expr_error():
+    proj = {
+        'name': 'module1', 'version': '1.0', 'description': 'x',
+        'license': 'LicenseRef-foo_bar',  # Underscore not allowed
+    }
+    with pytest.raises(config.ConfigError, match="can only contain"):
+        config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
+
+    proj['license'] = "BSD-33-Clause"  # Not a real license
+    with pytest.raises(config.ConfigError, match="recognised"):
+        config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
+
+
 def test_license_file_defaults_with_old_metadata():
     metadata = {'module': 'mymod', 'author': ''}
     info = config._prep_metadata(metadata, samples_dir / 'pep621_license_files' / 'pyproject.toml')

prepare_license_list.py

@@ -0,0 +1,24 @@
+# Call with path to SPDX license-list-data repo, cloned from:
+#   https://github.com/spdx/license-list-data
+
+import json
+import pprint
+import sys
+from pathlib import Path
+
+list_data_repo = Path(sys.argv[1])
+with (list_data_repo / 'json' / 'licenses.json').open('rb') as f:
+    licenses_json = json.load(f)
+
+condensed = {
+    l['licenseId'].lower() : {'id': l['licenseId']}
+    for l in licenses_json['licenses']
+    if not l['isDeprecatedLicenseId']
+}
+
+with Path('flit_core', 'flit_core', '_spdx_data.py').open('w') as f:
+    f.write("# This file is generated from SPDX license data; don't edit it manually.\n\n")
+
+    f.write("licenses = \\\n")
+    pprint.pprint(condensed, f)
+