Restricting the alphabet of a string

[Nathan Harmston]

Hi,

I ve being thinking about playing around with bit strings but use in
some encoding problems I m considering and was trying to decide how to
implement a bit string class. Is there a library out there for doing
basic things with bit strings already is my first question?

I know that I can extend string to bit string, but is there anyway I
can force the alphabet to be restricted to 1's and 0's (or even 1, 0
and -1, as an extension to from trinary strings).

class Binary_String(String):
    pass

Many Thanks

Nathan