def __getitem__(self, key: str) -> METRIC_DICT:
        """Supports getting calibrations by index.

        Calibration may be accessed by key:

            `calibration['t1']`.

        This returns a map from tuples of `cirq.GridQubit`s to a list of the
        values of the metric. If there are no targets, the only key will only
        be an empty tuple.
        """
        if not isinstance(key, str):
            raise TypeError(f'Calibration metrics only have string keys. Key was {key}')
        if key not in self._metric_dict:
            raise KeyError(f'Metric named {key} not in calibration')
        return self._metric_dict[key]
Supports getting calibrations by index.

        Calibration may be accessed by key:

            `calibration['t1']`.

        This returns a map from tuples of `cirq.GridQubit`s to a list of the
        values of the metric. If there are no targets, the only key will only
        be an empty tuple.
        
cirq-google/cirq_google/engine/calibration.py