gh-145492: Fix defaultdict __repr__ infinite recursion

Move Py_ReprLeave(dd->default_factory) inside the else branch so it is
only called when Py_ReprEnter returned 0 (successfully entered). When
Py_ReprEnter detects recursion (returns > 0), it does not add a new
entry to the repr tracking list. Calling Py_ReprLeave in that case
incorrectly removed the entry from the outer (non-recursive) call,
which allowed subsequent recursive calls to bypass the guard entirely,
leading to infinite recursion.

Includes a regression test.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mvanhorn

Lib/test/test_defaultdict.py

@@ -204,5 +204,20 @@ def default_factory():
         self.assertEqual(test_dict[key], 2)
         self.assertEqual(count, 2)
 
+    def test_repr_recursive_factory(self):
+        # gh-145492: defaultdict.__repr__ should not cause infinite recursion
+        # when the factory's __repr__ calls repr() on the defaultdict.
+        class ProblematicFactory:
+            def __call__(self):
+                return {}
+            def __repr__(self):
+                repr(dd)
+                return "ProblematicFactory()"
+
+        dd = defaultdict(ProblematicFactory())
+        # Should not raise RecursionError
+        r = repr(dd)
+        self.assertIn('ProblematicFactory()', r)
+
 if __name__ == "__main__":
     unittest.main()

Modules/_collectionsmodule.c

@@ -2385,9 +2385,10 @@ defdict_repr(PyObject *op)
             }
             defrepr = PyUnicode_FromString("...");
         }
-        else
+        else {
             defrepr = PyObject_Repr(dd->default_factory);
-        Py_ReprLeave(dd->default_factory);
+            Py_ReprLeave(dd->default_factory);
+        }
     }
     if (defrepr == NULL) {
         Py_DECREF(baserepr);