IBP banding data system: Internal maintenance specification

Because the class invariant states that the .spec4 attribute is always a four-letter code, flattening the code does not change its value. However, we do need to address the case of records with no species code, in which case the argument will be None.

# - - -   S p e c 4 F i e l d . f l a t t e n   - - -

    @staticmethod
    def flatten(spec4):
        '''Flatten a species code.
        '''
        if  spec4 is None:
            return  BLANK_SPEC4
        else:
            return  spec4.value