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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 cache_key(self, repo: str, branch: str, task: Task, git_repo: Repo) -> str: """ Returns the key used for storing results in cache. """

        return "{repo}_{branch}_{hash}_{task}".format(repo=self.repo_id(repo),
                                                      branch=branch,
                                                      hash=self.current_git_hash(repo, branch, git_repo),
                                                      task=task.hash)