[3.13] gh-116789: Add more tests for `inspect.getmembers` (GH-116802) (#123129)

gh-116789: Add more tests for `inspect.getmembers` (GH-116802)
(cherry picked from commit c15bfa9a71)

Co-authored-by: sobolevn <mail@sobolevn.me>

Lib/test/test_inspect/test_inspect.py

@@ -1553,6 +1553,56 @@ class TestClassesAndFunctions(unittest.TestCase):
         self.assertIn(('f', b.f), inspect.getmembers(b))
         self.assertIn(('f', b.f), inspect.getmembers(b, inspect.ismethod))
 
+    def test_getmembers_custom_dir(self):
+        class CorrectDir:
+            def __init__(self, attr):
+                self.attr = attr
+            def method(self):
+                return self.attr + 1
+            def __dir__(self):
+                return ['attr', 'method']
+
+        cd = CorrectDir(5)
+        self.assertEqual(inspect.getmembers(cd), [
+            ('attr', 5),
+            ('method', cd.method),
+        ])
+        self.assertEqual(inspect.getmembers(cd, inspect.ismethod), [
+            ('method', cd.method),
+        ])
+
+    def test_getmembers_custom_broken_dir(self):
+        # inspect.getmembers calls `dir()` on the passed object inside.
+        # if `__dir__` mentions some non-existent attribute,
+        # we still need to return others correctly.
+        class BrokenDir:
+            existing = 1
+            def method(self):
+                return self.existing + 1
+            def __dir__(self):
+                return ['method', 'missing', 'existing']
+
+        bd = BrokenDir()
+        self.assertEqual(inspect.getmembers(bd), [
+            ('existing', 1),
+            ('method', bd.method),
+        ])
+        self.assertEqual(inspect.getmembers(bd, inspect.ismethod), [
+            ('method', bd.method),
+        ])
+
+    def test_getmembers_custom_duplicated_dir(self):
+        # Duplicates in `__dir__` must not fail and return just one result.
+        class DuplicatedDir:
+            attr = 1
+            def __dir__(self):
+                return ['attr', 'attr']
+
+        dd = DuplicatedDir()
+        self.assertEqual(inspect.getmembers(dd), [
+            ('attr', 1),
+        ])
+
     def test_getmembers_VirtualAttribute(self):
         class M(type):
             def __getattr__(cls, name):