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Ben Slivinn
Ben Slivinn
10,156 Points

Could you please explain why are we using self = str.__new__(*args)?

Object-Oriented Python, Subclassing Built-ins.

I tried to play with ReversedStr's new() method and add a loop to accept multiple arguments (rs = ReversedStr(321, 987), and the first argument successfully converted to string type because of str.new() (In my understanding its because str.new()), but the second one isn't. That is when I understood that I don't really get it why you are using str.new().


class ReverseStr(str):
    def __new__(*args):
            self = str.__new__(*args) # self is just a variable name.
            self = self[::-1]
            return self

rs = ReverseStr(321)

Thank you!

1 Answer

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Great question!

From https://mail.python.org/pipermail/tutor/2008-April/061426.html:

Use __new__ when you need to control the creation of a new instance. Use __init__ when you need to control initialization of a new instance.

__new__ is the first step of instance creation. It's called first, and is responsible for returning a new instance of your class. In contrast, __init__ doesn't return anything; it's only responsible for initializing the instance after it's been created.

In general, you shouldn't need to override __new__ unless you're subclassing an immutable type like str, int, unicode or tuple.

The *args is used in the __new__ signature to allow an arbitrary number of arguments to be passed in. Using *args again on the call to str.__new__(*args) passes these arbitrary number of arguments on to the parent's method.

By using the call to str.__new__ you get all the parent class attributes and methods.

The __new__() method creates and returns exactly one object. It can not be used to create multiple objects.

Post back if you need more help. Good luck!!!