BUG: adjust to older pyproject-metadata releases

Author: dnicolodi

mesonpy/__init__.py

@@ -78,6 +78,8 @@ def canonicalize_license_expression(s: str) -> str:
 
 __version__ = '0.17.0.dev0'
 
+_PYPROJECT_METADATA_VERSION = tuple(map(int, pyproject_metadata.__version__.split('.')[:2]))
+_SUPPORTED_DYNAMIC_FIELDS = {'version', } if _PYPROJECT_METADATA_VERSION < (0, 9) else {'version', 'license', 'license-files'}
 
 _NINJA_REQUIRED_VERSION = '1.8.2'
 _MESON_REQUIRED_VERSION = '0.63.3' # keep in sync with the version requirement in pyproject.toml
@@ -272,7 +274,7 @@ def from_pyproject(
                 'Required "project.version" field is missing and not declared as dynamic')
 
         # Check for unsupported dynamic fields.
-        unsupported_dynamic = set(metadata.dynamic) - {'version', 'license', 'license-files'}
+        unsupported_dynamic = set(metadata.dynamic) - _SUPPORTED_DYNAMIC_FIELDS
         if unsupported_dynamic:
             fields = ', '.join(f'"{x}"' for x in unsupported_dynamic)
             raise pyproject_metadata.ConfigurationError(f'Unsupported dynamic fields: {fields}')
@@ -759,7 +761,12 @@ def __init__(
                 if license is None:
                     raise pyproject_metadata.ConfigurationError(
                         'Field "license" declared as dynamic but license is not specified in meson.build')
-                self._metadata.license = license
+                # mypy is not happy when analyzing typing based on
+                # pyproject-metadata < 0.9 where license needs to be of
+                # License type.  However, this code is not executed if
+                # pyproject-metadata is older than 0.9 because then dynamic
+                # license is not allowed.
+                self._metadata.license = license  # type: ignore[assignment]
             if 'license-files' in self._metadata.dynamic:
                 self._metadata.license_files = self._meson_license_files
         else:
@@ -768,9 +775,11 @@ def __init__(
             if version is None:
                 raise pyproject_metadata.ConfigurationError(
                     'Section "project" missing in pyproject.toml and version is not defined in meson.build')
-            self._metadata = Metadata(
-                name=name, version=packaging.version.Version(version),
-                license=self._meson_license, license_files=self._meson_license_files)
+            kwargs = {
+                'license': self._meson_license,
+                'license_files': self._meson_license_files
+            } if _PYPROJECT_METADATA_VERSION >= (0, 9) else {}
+            self._metadata = Metadata(name=name, version=packaging.version.Version(version), **kwargs)
 
         # verify that we are running on a supported interpreter
         if self._metadata.requires_python:
@@ -909,7 +918,7 @@ def _meson_license(self) -> Optional[str]:
         assert isinstance(value, str)
         if value == 'unknown':
             return None
-        return canonicalize_license_expression(value)
+        return str(canonicalize_license_expression(value)) # str() is to make mypy happy
 
     @property
     def _meson_license_files(self) -> Optional[List[pathlib.Path]]: