bpo-40126: Fix attribute out of scope during error handling in mock

Author: Barry McLarnon

Lib/unittest/mock.py

@@ -1244,6 +1244,7 @@ def decorate_callable(self, func):
  def patched(*args, **keywargs):
  extra_args = []
  entered_patchers = []
+ patching = None
 
  exc_info = tuple()
  try:

Misc/NEWS.d/next/Library/2020-04-03-21-15-27.bpo-40126.ggpdKD.rst

@@ -0,0 +1,4 @@
+Fixed an issue in unittest.mock where an unhandled exception can be raised
+by decorate_callable if patched.patchings is cleared during cleanup of
+another decorator. This causes the patching variable to be undefined in the
+exception handler, leading to an UnboundLocalError being raised.