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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 _repos_checked(self, worker, output, error):
 """Callback for _check_repos."""

        if worker.repo in self._checking_repos:
            self._checking_repos.remove(worker.repo)

        if output:
            self._valid_repos.append(worker.repo)

        if len(self._checking_repos) == 0:
            self._download_repodata(self._valid_repos)