Issue #10180: Pickling file objects is now explicitly forbidden, since

unpickling them produced nonsensical results.

Lib/_pyio.py

@@ -747,6 +747,10 @@ class _BufferedIOMixin(BufferedIOBase):
     def mode(self):
         return self.raw.mode
 
+    def __getstate__(self):
+        raise TypeError("can not serialize a '{0}' object"
+                        .format(self.__class__.__name__))
+
     def __repr__(self):
         clsname = self.__class__.__name__
         try: