Extract Python 3.9 compatibility layer into its own file

Author: abravalheri

importlib_metadata/__init__.py

@@ -14,7 +14,7 @@
 import posixpath
 import collections
 
-from . import _adapters, _meta
+from . import _adapters, _meta, _py39compat
 from ._collections import FreezableDefaultDict, Pair
 from ._compat import (
     NullFinder,
@@ -29,7 +29,7 @@
 from importlib import import_module
 from importlib.abc import MetaPathFinder
 from itertools import starmap
-from typing import List, Mapping, Optional, Tuple, Union
+from typing import List, Mapping, Optional, Union
 
 
 __all__ = [
@@ -382,8 +382,8 @@ def select(self, **params):
         Select entry points from self that match the
         given parameters (typically group and/or name).
         """
-        candidates = (_ep_matches(ep, **params) for ep in self)
-        return EntryPoints(ep for ep, ep_matches in candidates if ep_matches)
+        candidates = (_py39compat.ep_matches(ep, **params) for ep in self)
+        return EntryPoints(ep for ep, predicate in candidates if predicate)
 
     @property
     def names(self):
@@ -415,15 +415,6 @@ def _from_text(text):
         )
 
 
-def _ep_matches(ep: EntryPoint, **params) -> Tuple[EntryPoint, bool]:
-    """Compatibility layer for EntryPoint objects in Python 3.8/3.9 stdlib."""
-    try:
-        return ep, ep.matches(**params)
-    except AttributeError:
-        _ep = EntryPoint(ep.name, ep.value, ep.group)
-        return _ep, _ep.matches(**params)
-
-
 class Deprecated:
     """
     Compatibility add-in for mapping to indicate that
@@ -1029,21 +1020,9 @@ def version(distribution_name):
     return distribution(distribution_name).version
 
 
-def _compat_normalized_name(dist: Distribution) -> Optional[str]:
-    """
-    Compatibility shim to honor name normalization for distributions
-    that don't provide ``_normalized_name``
-    (as in ``importlib.metadata`` for Python 3.8/3.9).
-    """
-    try:
-        return dist._normalized_name
-    except AttributeError:
-        return Prepared.normalize(getattr(dist, "name", None) or dist.metadata['Name'])
-
-
 _unique = functools.partial(
     unique_everseen,
-    key=_compat_normalized_name,
+    key=_py39compat.normalized_name,
 )
 """
 Wrapper for ``distributions`` to return unique distributions by name.

importlib_metadata/_py39compat.py

@@ -0,0 +1,48 @@
+"""
+Compatibility layer with Python 3.8/3.9
+"""
+from typing import TYPE_CHECKING, Any, Optional, Tuple
+
+
+if TYPE_CHECKING:
+    from . import Distribution, EntryPoint
+else:
+    Distribution = EntryPoint = Any
+
+
+def normalized_name(dist: Distribution) -> Optional[str]:
+    """
+    Honor name normalization for distributions that don't provide ``_normalized_name``.
+    """
+    try:
+        return dist._normalized_name
+    except AttributeError:
+        from . import Prepared
+
+        return Prepared.normalize(getattr(dist, "name", None) or dist.metadata['Name'])
+
+
+def ep_matches(ep: EntryPoint, **params) -> Tuple[EntryPoint, bool]:
+    """
+    Workaround for ``EntryPoint`` objects without the ``matches`` method.
+    For the sake of convenience, a tuple is returned containing not only the
+    boolean value corresponding to the predicate evalutation, but also a compatible
+    ``EntryPoint`` object that can be safely used at a later stage.
+
+    For example, the following sequences of expressions should be compatible:
+
+        # Sequence 1: using the compatibility layer
+        candidates = (_py39compat.ep_matches(ep, **params) for ep in entry_points)
+        [ep for ep, predicate in candidates if predicate]
+
+        # Sequence 2: using Python 3.9+
+        [ep for ep in entry_points if ep.matches(**params)]
+    """
+    try:
+        return ep, ep.matches(**params)
+    except AttributeError:
+        from . import EntryPoint
+
+        # Reconstruct the EntryPoint object to make sure it is compatible.
+        _ep = EntryPoint(ep.name, ep.value, ep.group)
+        return _ep, _ep.matches(**params)