Prefer valid distributions when collecting entry points.

Where an unclean upgrade leaves an empty metadata directory beside the valid
one, `_unique` kept whichever the file system happened to list first, so the
entry points of the valid distribution could go missing entirely.

Closes #534.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: jaraco

importlib_metadata/__init__.py

@@ -1224,7 +1224,8 @@ def entry_points(**params) -> EntryPoints:
     :return: EntryPoints for all installed packages.
     """
     eps = itertools.chain.from_iterable(
-        dist.entry_points for dist in _unique(distributions())
+        dist.entry_points
+        for dist in _unique(Distribution._prefer_valid(distributions()))
     )
     return EntryPoints(eps).select(**params)
 

newsfragments/534.bugfix.rst

@@ -0,0 +1,5 @@
+``entry_points()`` now prefers distributions whose metadata is readable, so
+an empty metadata directory left behind by an unclean upgrade no longer
+shadows the valid one of the same name and suppresses its entry points.
+Invalid distributions are deferred to the end of the enumeration rather than
+dropped.

tests/test_main.py

@@ -160,6 +160,40 @@ def test_valid_dists_preferred(self):
         dist = Distribution.from_name('foo')
         assert dist.version == "1.0"
 
+    def test_invalid_dists_deferred(self):
+        """
+        Preferring the valid dists defers the invalid ones to the end
+        rather than dropping them.
+        """
+        fixtures.build_files(self.make_pkg('foo-4.0', files={}), self.site_dir)
+        fixtures.build_files(self.make_pkg('foo-4.1'), self.site_dir)
+        dists = list(Distribution._prefer_valid(Distribution.discover(name='foo')))
+        assert len(dists) == 2
+        assert dists[0].version == "1.0"
+
+    def test_entry_points_from_valid_dist(self):
+        """
+        Entry points come from the valid dist even when an invalid one
+        of the same name is discovered first.
+
+        Ref python/importlib_metadata#534.
+        """
+        # as in test_valid_dists_preferred, bracket the valid dist so that
+        # on most file systems it is never naturally first.
+        fixtures.build_files(self.make_pkg('foo-4.0', files={}), self.site_dir)
+        fixtures.build_files(
+            self.make_pkg(
+                'foo-4.1',
+                files={
+                    'METADATA': "VERSION: 1.0",
+                    'entry_points.txt': '[shadowed]\nep = foo:main\n',
+                },
+            ),
+            self.site_dir,
+        )
+        fixtures.build_files(self.make_pkg('foo-4.2', files={}), self.site_dir)
+        assert entry_points(group='shadowed').names == {'ep'}
+
     def test_missing_metadata(self):
         """
         Dists with a missing metadata file should raise ``MetadataNotFound``.