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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 function_dependency_graph(self, func):
 """ Get a dependency graph for the function `func`. :param func: The Function object in CFG.function_manager. :returns: A networkx.DiGraph instance. """


        if self._function_data_dependencies is None:
            self._build_function_dependency_graphs()

        if func in self._function_data_dependencies:
            return self._function_data_dependencies[func]

        # Not found
        return None