Compute the standard deviation of an array, ignoring NaNs.

        :param a: The array.
        :return: The standard deviation of the array.

def fn_std(self, a, axis=None):
        """
        Compute the standard deviation of an array, ignoring NaNs.

        :param a: The array.
        :return: The standard deviation of the array.
        """

        return numpy.nanstd(self._to_ndarray(a), axis=axis)