ABC for String?

I recently discovered abstract base classes (ABCs) in collections and like their clear, systematic approach and Mixins. Now I also want to create customs strings (*), but I couldn't find 开发者_开发问答an ABC for strings.

There is UserString, but UserDict was discouraged!? Deriving from str itself would have no Mixins? How would you access the "data" part of the string in overridden methods?

Somewhere I saw the suggestions to derive from Sequence and Hashable, but then I couldn't write if 'test' in my_string:?!

Which approach do you recommend?

(*) The reasons are: - write strings that order in an internally defined way - make string (as part of an enumeration), that throw errors when comparing to values outside the enumeration scope

Here's a silly, but quick, example of Steven's answer. It's implemented in Python 3 (i.e. Unicode strings, super without arguments, and __getitem__ slices):

class MultiStr(str):
    def __new__(cls, string, multiplier=1, **kwds):
        self = super().__new__(cls, string, **kwds)
        self.multiplier = multiplier
        return self

    def __getitem__(self, index):
        item = super().__getitem__(index)
        return item * self.multiplier

>>> s = MultiStr(b'spam', multiplier=3, encoding='ascii')
>>> s[0]
'sss'
>>> s[:2]
'spspsp'
>>> s[:]
'spamspamspam'

You can just subclass str, you wouldn't need any mixins because you inherit everything you need from str itself. As for the "data" part: as you're not "simulating" a string (which is what you'd use UserString for), there is no need for a separate "data" part, use the string itself (that is: use self as you would use a string).

(if you mean something else: maybe the question would be clearer by showing your (attempted) code for the overridden methods)