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

Question

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().
Sourcecode:
```Python
class ReverseStr(str):
    def new(args):
            self = str.new(args) # self is just a variable name.
            self = self[::-1]
            return self
rs = ReverseStr(321)
print(rs)
```
Thank you!

Chris Freeman · Accepted Answer

Great question!
From https://mail.python.org/pipermail/tutor/2008-April/061426.html (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 (https://docs.python.org/3/library/stdtypes.html#string-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!!!

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Ben Slivinn

Ben Slivinn

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

1 Answer

Chris Freeman

Chris Freeman