From: Matt Van Horn <mvanhorn@users.noreply.github.com>
Date: Tue, 10 Mar 2026 13:20:42 +0000 (-0700)
Subject: gh-145492: Fix defaultdict __repr__ infinite recursion (GH-145659)
X-Git-Tag: v3.15.0a8~379^2~2
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2d35f9bc1cf61b27639ed992dfbf363ab436fd8b;p=thirdparty%2FPython%2Fcpython.git

gh-145492: Fix defaultdict __repr__ infinite recursion (GH-145659)

Co-Authored-By: Thomas Kowalski <thom.kowa@gmail.com>
---

diff --git a/Lib/test/test_defaultdict.py b/Lib/test/test_defaultdict.py
index fbd7354a915a..732e9a876ca8 100644
--- a/Lib/test/test_defaultdict.py
+++ b/Lib/test/test_defaultdict.py
@@ -204,5 +204,20 @@ class TestDefaultDict(unittest.TestCase):
         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()
diff --git a/Misc/NEWS.d/next/Library/2026-03-09-00-00-00.gh-issue-145492.457Afc.rst b/Misc/NEWS.d/next/Library/2026-03-09-00-00-00.gh-issue-145492.457Afc.rst
new file mode 100644
index 000000000000..297ee4099f12
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-03-09-00-00-00.gh-issue-145492.457Afc.rst
@@ -0,0 +1,3 @@
+Fix infinite recursion in :class:`collections.defaultdict` ``__repr__``
+when a ``defaultdict`` contains itself. Based on analysis by KowalskiThomas
+in :gh:`145492`.
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index c3d63c8aab4b..15c9aa419118 100644
--- a/Modules/_collectionsmodule.c
+++ b/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);