def create_from_datatype(cls, datatype, field_name, label=None):
    """
        create_from_datatype returns a FilterControl subclass, based on the datatype.  If datatype matches a registered
        datatype, then the appropriate subclass is instantiated and returned.

        :param datatype: The datatype of the field.  A string.
        :param field_name:  The name of the FilterControl subclass - passed thru to the constructor
        :param label:  The label for the FilterControl subclass - passed through to the constructor
        :return:  A FilterControl subclass.
        """
    for test, factory, dt in cls.filter_controls:
        if dt == datatype:
            return factory(field_name, label)
