From 330b18f2fe4693c08931ec9295904a036b7456d0 Mon Sep 17 00:00:00 2001
From: Changjoon <sisyphus9402@gmail.com>
Date: Wed, 29 Apr 2026 18:14:32 +0900
Subject: [PATCH] Preserve recursively-set value in defaultdict.__missing__
 (#7718)

CPython's defaultdict.__missing__ (Modules/_collectionsmodule.c::defdict_missing)
calls default_factory() first; if the factory's recursion already populated
self[key] while running, the existing value is preserved instead of being
overwritten.

RustPython ships a Python fallback at Lib/collections/_defaultdict.py
(the C _collections.defaultdict is not available). That fallback
unconditionally executed self[key] = val after the factory returned,
overwriting any value the recursive call had already stored.

Add a 'if key in self: return self[key]' guard before the assignment.
dict.__contains__ does not invoke __missing__, so there's no recursion
risk; in the common non-reentrant case the check is False and behavior
is unchanged.

Unmasks test_defaultdict.TestDefaultDict.test_factory_conflict_with_set_value.
---
 Lib/collections/_defaultdict.py | 4 ++++
 Lib/test/test_defaultdict.py    | 1 -
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/Lib/collections/_defaultdict.py b/Lib/collections/_defaultdict.py
index b9c6c4961..cb9d403c8 100644
--- a/Lib/collections/_defaultdict.py
+++ b/Lib/collections/_defaultdict.py
@@ -17,6 +17,10 @@ class defaultdict(dict):
             val = self.default_factory()
         else:
             raise KeyError(key)
+        # CPython parity: a recursive __missing__ via factory() may have
+        # already populated key; preserve that value instead of overwriting.
+        if key in self:
+            return self[key]
         self[key] = val
         return val
 
diff --git a/Lib/test/test_defaultdict.py b/Lib/test/test_defaultdict.py
index f10603fb4..fbd7354a9 100644
--- a/Lib/test/test_defaultdict.py
+++ b/Lib/test/test_defaultdict.py
@@ -186,7 +186,6 @@ class TestDefaultDict(unittest.TestCase):
         with self.assertRaises(TypeError):
             i |= None
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; AssertionError: 1 != 2
     def test_factory_conflict_with_set_value(self):
         key = "conflict_test"
         count = 0