trouble subclassing str

[Brent]

I'd like to subclass the built-in str type.  For example:

--

class MyString(str):

    def __init__(self, txt, data):
        super(MyString,self).__init__(txt)
        self.data = data

if __name__ == '__main__':

    s1 = MyString("some text", 100)

--

but I get the error:

Traceback (most recent call last):
  File "MyString.py", line 27, in ?
    s1 = MyString("some text", 12)
TypeError: str() takes at most 1 argument (2 given)

I am using Python 2.3 on OS X.  Ideas?