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This is because when an subprocess.CalledProcessError is raised, the CalledProcessError instance is picked through a socket and then read by "parent" process, but in fact CalledProcessError instances cannot be picked correctly. Because CalledProcessError extends PyExc_BaseException, which defines a __reduce__ method, that special method cause the pickle load to call the exception type's __init__ method with packed self.args as arguments. So if a subclass of "Exception" needs to behave correctly in pickling, it should make self.args meats its __init__ method's function signature. That is, ensure the calling to:
 self.__init__(*self.args)
has no problem. 
But CalledProcessError doesn't meat this requirement. Here is a patch fixing the CalledProcessError.__init__ to call its base class's __init__ to store its arguments in self.args, which can fix this bug.