ADT for restricted set of values

[Graham Fawcett]

Ben Finney wrote:
> Howdy all,
>
> I'd like to have an Abstract Data Type for a scalar value that is
> restricted to a small set of values. Like an Enum, I suppose.
>
> What I would like is to be able to use simple 'str' values in most of
> the code, but where the values are actually used in a semantically
> meaningful context, have them used as an ADT that will barf if the
> value isn't part of the set.

How about this:

class BaseEnum(str):
    """Base class for enumerations. You must subclass and provide
    values for the _valid list."""

    _valid = []

    def __init__(self, v):
        assert self in self._valid, 'value %r not in %r' % (self,
self._valid)

cass Gender(BaseEnum):
    _valid = ['male', 'female', 'unknown']

Trying the following:

Gender('male')
Gender('female')
assert Gender('male') == 'male'
Gender('shemale')

The last will return an error:
AssertionError: value 'shemale' not in ['male', 'female', 'unknown']

Graham