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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 hook(self, id_num):
 """Get a single hook. :param int id_num: (required), id of the hook :returns: :class:`Hook <github3.repos.hook.Hook>` if successful, otherwise None """

        json = None
        if int(id_num) > 0:
            url = self._build_url('hooks', str(id_num), base_url=self._api)
            json = self._json(self._get(url), 200)
        return Hook(json, self) if json else None