diff -r 37a07cee5969 Lib/unittest/mock.py
--- a/Lib/unittest/mock.py	Sat Dec 05 17:05:23 2015 -0800
+++ b/Lib/unittest/mock.py	Sun Jan 17 20:31:07 2016 +0900
@@ -444,6 +444,7 @@
                        _eat_self=False):
         _spec_class = None
         _spec_signature = None
+        _spec_code = None
 
         if spec is not None and not _is_list(spec):
             if isinstance(spec, type):
@@ -453,6 +454,8 @@
             res = _get_signature_object(spec,
                                         _spec_as_instance, _eat_self)
             _spec_signature = res and res[1]
+            if inspect.isfunction(spec):
+                _spec_code = spec.__code__
 
             spec = dir(spec)
 
@@ -461,6 +464,7 @@
         __dict__['_spec_set'] = spec_set
         __dict__['_spec_signature'] = _spec_signature
         __dict__['_mock_methods'] = spec
+        __dict__['__code__'] = _spec_code
 
 
     def __get_return_value(self):