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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 _ensure_ctable(self, sid):
 """Ensure that a ctable exists for ``sid``, then return it."""

        sidpath = self.sidpath(sid)
        if not os.path.exists(sidpath):
            return self._init_ctable(sidpath)
        return bcolz.ctable(rootdir=sidpath, mode='a')