def flatten_json(nested_json):
        """Flatten json file with nested keys into a single level.

        Args:
            nested_json: A nested json file.

        Returns:
            The flattened json file if successful, None otherwise.
        """
        out = {}

        def flatten(x, name=''):
            if type(x) is dict:
                for a in x:
                    flatten(x[a], name + a + '_')
            elif type(x) is list:
                i = 0
                for a in x:
                    flatten(a, name + str(i) + '_')
                    i += 1
            else:
                out[name[:-1]] = x

        flatten(nested_json)

        return out