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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 get_download_url(self, proapi=False):
 """ Get this file's download URL :param bool proapi: whether to use pro API """

        if self._download_url is None:
            self._download_url = \
                self.api._req_files_download_url(self.pickcode, proapi)
        return self._download_url