Issue #119600: mock: do not access attributes of original when new_callable is set

In order to patch flask.g e.g. as in #84982, that
proxies getattr must not be invoked. For that,
mock must not try to read from the original
object. In some cases that is unavoidable, e.g.
when doing autospec. However, patch("flask.g",
new_callable=MagicMock) should be entirely safe.

Author: rbtcollins

Lib/unittest/mock.py

@@ -1508,13 +1508,12 @@ def __enter__(self):
                 if isinstance(original, type):
                     # If we're patching out a class and there is a spec
                     inherit = True
-            if spec is None and _is_async_obj(original):
-                Klass = AsyncMock
-            else:
-                Klass = MagicMock
-            _kwargs = {}
+
+            # Determine the Klass to use
             if new_callable is not None:
                 Klass = new_callable
+            elif spec is None and _is_async_obj(original):
+                Klass = AsyncMock
             elif spec is not None or spec_set is not None:
                 this_spec = spec
                 if spec_set is not None:
@@ -1527,7 +1526,10 @@ def __enter__(self):
                     Klass = AsyncMock
                 elif not_callable:
                     Klass = NonCallableMagicMock
+            else:
+                Klass = MagicMock
 
+            _kwargs = {}
             if spec is not None:
                 _kwargs['spec'] = spec
             if spec_set is not None: