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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 _format_index(self, volume_system, idx):
 """Returns a formatted index given the disk index idx."""


        if volume_system.parent.index is not None:
            return '{0}.{1}'.format(volume_system.parent.index, idx)
        else:
            return str(idx)