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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 _inspect_disk(self):
 """Inspects the disk and returns the mountpoints mapping as a list which order is the supposed one for correct mounting. """

        roots = self._handler.inspect_os()

        if roots:
            self._root = roots[0]
            return sorted(self._handler.inspect_get_mountpoints(self._root),
                          key=lambda m: len(m[0]))
        else:
            raise RuntimeError("No OS found on the given disk image.")