gh-123934: Fix MagicMock not to reset magic method return values (#124038)

Lib/unittest/mock.py

@@ -628,7 +628,7 @@ class NonCallableMock(Base):
     side_effect = property(__get_side_effect, __set_side_effect)
 
 
-    def reset_mock(self,  visited=None,*, return_value=False, side_effect=False):
+    def reset_mock(self, visited=None, *, return_value=False, side_effect=False):
         "Restore the mock object to its initial state."
         if visited is None:
             visited = []
@@ -2218,6 +2218,17 @@ class MagicMock(MagicMixin, Mock):
         self._mock_add_spec(spec, spec_set)
         self._mock_set_magics()
 
+    def reset_mock(self, /, *args, return_value=False, **kwargs):
+        if (
+            return_value
+            and self._mock_name
+            and _is_magic(self._mock_name)
+        ):
+            # Don't reset return values for magic methods,
+            # otherwise `m.__str__` will start
+            # to return `MagicMock` instances, instead of `str` instances.
+            return_value = False
+        super().reset_mock(*args, return_value=return_value, **kwargs)
 
 
 class MagicProxy(Base):