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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 pull_request(self, number):
 """Get the pull request indicated by ``number``. :param int number: (required), number of the pull request. :returns: :class:`PullRequest <github3.pulls.PullRequest>` """

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