How to subclass str in Python

Question

I am trying to subclass str object, and add couple of methods to it. My main purpose is to learn how to do it. Where I am stuck is, am I supposed to subclass string in a met

Answer 1

Here's a quick hack to do what you want: you basically intercept every function call, and, if you see that it's returning a string, you convert it back to your own class type.

While this works in this simple example, it has some limitations. Among other things, operators such as the subscript operator are apparently not handled.

class FunWrapper(object):
    def __init__(self, attr):
        self.attr = attr

    def __call__(self, *params, **args):
        ret = self.attr(*params, **args)
        if type(ret) is str:
            return Foo(ret)
        return ret

class Foo(object):
    def __init__(self, string):
        self.string = string

    def __getattr__(self, attr):
        return FunWrapper(getattr(self.string, attr))

    def newMethod(self):
        return "*%s*" % self.string.upper()


f = Foo('hello')
print f.upper().newMethod().lower()