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

I am trying to subclass str object, and add couple of methods to it. My main purpose is to learn how to do it.

UserString was created before it was possible to subclass str directly, so prefer to subclass str, instead of using UserString (as another answer suggests).

When subclassing immutable objects, it's usually necessary to modify the data before you instantiate the object - therefore you need to both implement __new__ and call the parent __new__ (preferably with super, instead of str.__new__ as another answer suggests).

In Python 3, it is more performant to call super like this:

class Caps(str):
    def __new__(cls, content):
        return super().__new__(cls, content.upper())

__new__ looks like a class method, but it is actually implemented as a static method, so we need to pass cls redundantly as the first argument. We don't need the @staticmethod decorator, however.

If we use super like this to support Python 2, we'll note the redundant cls more clearly:

class Caps(str):
    def __new__(cls, content):
        return super(Caps, cls).__new__(cls, content.upper())

Usage:

>>> Caps('foo')
'FOO'
>>> isinstance(Caps('foo'), Caps)
True
>>> isinstance(Caps('foo'), str)
True

The complete answer

None of the answers so far does what you've requested here:

My class's methods, should be completely chainable with str methods, and should always return a new my class instance when custom methods modified it. I want to be able to do something like this:

a = mystr("something")
b = a.lower().mycustommethod().myothercustommethod().capitalize()
issubclass(b,mystr) # True

(I believe you mean isinstance(), not issubclass().)

You need a way to intercept the string methods. __getattribute__ does emphasized textthis.

class Caps(str):
    def __new__(cls, content):
        return super(Caps, cls).__new__(cls, content.upper())
    def __repr__(self):
        """A repr is useful for debugging"""
        return f'{type(self).__name__}({super().__repr__()})'
    def __getattribute__(self, name):
        if name in dir(str): # only handle str methods here
            def method(self, *args, **kwargs):
                value = getattr(super(), name)(*args, **kwargs)
                # not every string method returns a str:
                if isinstance(value, str):
                    return type(self)(value)  
                elif isinstance(value, list):
                    return [type(self)(i) for i in value]
                elif isinstance(value, tuple):
                    return tuple(type(self)(i) for i in value)
                else: # dict, bool, or int
                    return value
            return method.__get__(self) # bound method 
        else: # delegate to parent
            return super().__getattribute__(name)
    def mycustommethod(self): # shout
        return type(self)(self + '!')
    def myothercustommethod(self): # shout harder
        return type(self)(self + '!!')

and now:

>>> a = Caps("something")
>>> a.lower()
Caps('SOMETHING')
>>> a.casefold()
Caps('SOMETHING')
>>> a.swapcase()
Caps('SOMETHING')
>>> a.index('T')
4
>>> a.strip().split('E')
[Caps('SOM'), Caps('THING')]

And the case requested works:

>>> a.lower().mycustommethod().myothercustommethod().capitalize()
Caps('SOMETHING!!!')

Response to Comment

Why is the Python 3 only call, i.e. super().method(arg) more performant?

The function already has access to both __class__ and self without doing a global and local lookup:

class Demo:
    def foo(self):
        print(locals())
        print(__class__)

>>> Demo().foo()
{'self': <__main__.Demo object at 0x7fbcb0485d90>, '__class__': }

See the source for more insight.