Usage to call a method of an instance before it is returned by __new__

__new__ is a constructor that creates a new instance of that class, and __init__ is, as the name implies, an initializer for customizing a newly created instance. This official Python documentに説明されているとおり、__init__は、__new__が作成したインスタンスを、呼び出し元に返される前にselfとして受け取ります。

In fact, it seems possible to use the instance method at that point, even inside __new__, if the instance has already been created.

In the following sample code, immediately after creating an instance of Foo using ʻobject .__ new in Foo . new __, immediately call the ʻearly_call instance method and set the instance variable self.arg I have created it, but I am able to execute it properly.

python

class Foo:

    def __new__(cls, arg):
        instance = object.__new__(cls)
        instance.early_call(arg)
        return instance
                 
    def __init__(self, arg):
        print(self.arg)

    def early_call(self, arg):
        self.arg = arg

foo = Foo("I'm foo.")    # I'm foo.Is output.

In fact, the implementation of pandas MultiIndex uses this usage. I will.