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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def method_double_for(self, method_name):
 """Returns the method double for the provided method name, creating one if necessary. :param str method_name: The name of the method to retrieve a method double for. :return: The mapped ``MethodDouble``. :rtype: MethodDouble """


        if method_name not in self._method_doubles:
            self._method_doubles[method_name] = MethodDouble(method_name, self._target)

        return self._method_doubles[method_name]