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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 compare_commits(self, base, head):
 """Compare two commits. :param str base: (required), base for the comparison :param str head: (required), compare this against base :returns: :class:`Comparison <github3.repos.comparison.Comparison>` if successful, else None """

        url = self._build_url('compare', base + '...' + head,
                              base_url=self._api)
        json = self._json(self._get(url), 200)
        return Comparison(json) if json else None