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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 timers(self, filename=None, mpi_rank="0"):
 """ Return the list of timers associated to the given `filename` and MPI rank mpi_rank. """

        if filename is not None:
            return [self._timers[filename][mpi_rank]]
        else:
            return [self._timers[filename][mpi_rank] for filename in self._filenames]