Issue 11875: Keep OrderedDict's __reduce__ from temporarily mutating the object.

Lib/collections.py

@@ -115,10 +115,9 @@ class OrderedDict(dict):
     def __reduce__(self):
         'Return state information for pickling'
         items = [[k, self[k]] for k in self]
-        tmp = self.__map, self.__root
-        del self.__map, self.__root
         inst_dict = vars(self).copy()
-        self.__map, self.__root = tmp
+        for k in vars(OrderedDict()):
+            inst_dict.pop(k, None)
         if inst_dict:
             return (self.__class__, (items,), inst_dict)
         return self.__class__, (items,)

Misc/NEWS

@@ -59,6 +59,9 @@ Library
   Py_AddPendingCall() for the first signal to fix a deadlock on reentrant or
   parallel calls. PyErr_SetInterrupt() writes also into the wake up file.
 
+- Issue #11875: collections.OrderedDict's __reduce__ was temporarily
+  mutating the object instead of just working on a copy.
+
 - Issue #11442: Add a charset parameter to the Content-type in SimpleHTTPServer
   to avoid XSS attacks.