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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 default_branch(self, file):
 """ Decide the name of the default branch given the file and the configuration :param file: File with informations about it :return: Branch Name """

        if isinstance(self.__default_branch__, str):
            return self.__default_branch__
        elif self.__default_branch__ == GithubProxy.DEFAULT_BRANCH.NO:
            return self.master_upstream
        else:
            return file.sha[:8]