def get_freq(self):
        """Find the appropriate frequency string to describe the inferred frequency of self.index

        Adapted from `pandas.tsseries.frequencies._FrequencyInferer.get_freq` for CFTimeIndexes.

        Returns
        -------
        str or None
        """
        if not self.is_monotonic or not self.index.is_unique:
            return None

        delta = self.deltas[0]  # Smallest delta
        if _is_multiple(delta, _ONE_DAY):
            return self._infer_daily_rule()
        # There is no possible intraday frequency with a non-unique delta
        # Different from pandas: we don't need to manage DST and business offsets in cftime
        elif len(self.deltas) != 1:
            return None

        if _is_multiple(delta, _ONE_HOUR):
            return _maybe_add_count("h", delta / _ONE_HOUR)
        elif _is_multiple(delta, _ONE_MINUTE):
            return _maybe_add_count("min", delta / _ONE_MINUTE)
        elif _is_multiple(delta, _ONE_SECOND):
            return _maybe_add_count("s", delta / _ONE_SECOND)
        elif _is_multiple(delta, _ONE_MILLI):
            return _maybe_add_count("ms", delta / _ONE_MILLI)
        else:
            return _maybe_add_count("us", delta / _ONE_MICRO)
Find the appropriate frequency string to describe the inferred frequency of self.index

        Adapted from `pandas.tsseries.frequencies._FrequencyInferer.get_freq` for CFTimeIndexes.

        Returns
        -------
        str or None
        
xarray/coding/frequencies.py