Refactor the casting into a wrapper for brevity and to document its purpose.

Author: jaraco

importlib_metadata/__init__.py

@@ -27,7 +27,7 @@
 from importlib import import_module
 from importlib.abc import MetaPathFinder
 from itertools import starmap
-from typing import Any, cast
+from typing import Any
 
 from . import _meta
 from ._collections import FreezableDefaultDict, Pair
@@ -38,6 +38,7 @@
 from ._functools import method_cache, pass_none
 from ._itertools import always_iterable, bucket, unique_everseen
 from ._meta import PackageMetadata, SimplePath
+from ._typing import md_none
 from .compat import py39, py311
 
 __all__ = [
@@ -543,7 +544,7 @@ def _assemble_message(text: str) -> _meta.PackageMetadata:
     @property
     def name(self) -> str:
         """Return the 'Name' metadata for the distribution package."""
-        return cast(PackageMetadata, self.metadata)['Name']
+        return md_none(self.metadata)['Name']
 
     @property
     def _normalized_name(self):
@@ -553,7 +554,7 @@ def _normalized_name(self):
     @property
     def version(self) -> str:
         """Return the 'Version' metadata for the distribution package."""
-        return cast(PackageMetadata, self.metadata)['Version']
+        return md_none(self.metadata)['Version']
 
     @property
     def entry_points(self) -> EntryPoints:
@@ -1125,7 +1126,7 @@ def packages_distributions() -> Mapping[str, list[str]]:
     pkg_to_dist = collections.defaultdict(list)
     for dist in distributions():
         for pkg in _top_level_declared(dist) or _top_level_inferred(dist):
-            pkg_to_dist[pkg].append(cast(PackageMetadata, dist.metadata)['Name'])
+            pkg_to_dist[pkg].append(md_none(dist.metadata)['Name'])
     return dict(pkg_to_dist)
 
 

importlib_metadata/_typing.py

@@ -0,0 +1,15 @@
+import functools
+import typing
+
+from ._meta import PackageMetadata
+
+md_none = functools.partial(typing.cast, PackageMetadata)
+"""
+Suppress type errors for optional metadata.
+
+Although Distribution.metadata can return None when metadata is corrupt
+and thus None, allow callers to assume it's not None and crash if
+that's the case.
+
+# python/importlib_metadata#493
+"""

importlib_metadata/compat/py39.py

@@ -4,15 +4,15 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any, cast
+from typing import TYPE_CHECKING, Any
 
 if TYPE_CHECKING:  # pragma: no cover
     # Prevent circular imports on runtime.
     from .. import Distribution, EntryPoint
 else:
     Distribution = EntryPoint = Any
 
-from .._meta import PackageMetadata
+from .._typing import md_none
 
 
 def normalized_name(dist: Distribution) -> str | None:
@@ -25,7 +25,7 @@ def normalized_name(dist: Distribution) -> str | None:
         from .. import Prepared  # -> delay to prevent circular imports.
 
         return Prepared.normalize(
-            getattr(dist, "name", None) or cast(PackageMetadata, dist.metadata)['Name']
+            getattr(dist, "name", None) or md_none(dist.metadata)['Name']
         )