gh-105080: Fixed inconsistent signature on derived classes (#105217)

1 files changed, 12 insertions, 11 deletions

diff --git a/Lib/inspect.py b/Lib/inspect.py
index 15eefdb..a550202 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -2581,17 +2581,18 @@ def _signature_from_callable(obj, *,
             factory_method = None
             new = _signature_get_user_defined_method(obj, '__new__')
             init = _signature_get_user_defined_method(obj, '__init__')
-            # Now we check if the 'obj' class has an own '__new__' method
-            if '__new__' in obj.__dict__:
-                factory_method = new
-            # or an own '__init__' method
-            elif '__init__' in obj.__dict__:
-                factory_method = init
-            # If not, we take inherited '__new__' or '__init__', if present
-            elif new is not None:
-                factory_method = new
-            elif init is not None:
-                factory_method = init
+
+            # Go through the MRO and see if any class has user-defined
+            # pure Python __new__ or __init__ method
+            for base in obj.__mro__:
+                # Now we check if the 'obj' class has an own '__new__' method
+                if new is not None and '__new__' in base.__dict__:
+                    factory_method = new
+                    break
+                # or an own '__init__' method
+                elif init is not None and '__init__' in base.__dict__:
+                    factory_method = init
+                    break
 
             if factory_method is not None:
                 sig = _get_signature_of(factory_method)