@staticmethod
def _check_key_valid(key):
    """_check_key_valid.

        Internal method to check if an input key to the dictionary is valid.
        Checks to see if ``key`` is a tuple.

        Parameters
        ----------
        key : anything, but must be a tuple to be valid.

        Returns
        -------
        None.

        Raises
        ------
        KeyError if the key is invalid.

        """
    if not isinstance(key, tuple):
        raise KeyError('Key formatted incorrectly, must be tuple')
