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in Python by (1.6k points)

I'm just trying to streamline one of my classes and have introduced some functionality in the same style as the flyweight design pattern.

However, I'm a bit confused as to why __init__ is always called after __new__. I wasn't expecting this. Can anyone tell me why this is happening and how I can implement this functionality otherwise? (Apart from putting the implementation into the __new__ which feels quite hacky.)

Here's an example:

class A(object):

    _dict = dict()

    def __new__(cls):

        if 'key' in A._dict:

            print "EXISTS"

            return A._dict['key']

        else:

            print "NEW"

            return super(A, cls).__new__(cls)

    def __init__(self):

        print "INIT"

        A._dict['key'] = self

        print ""

a1 = A()

a2 = A()

a3 = A()

Outputs:

NEW

INIT

EXISTS

INIT

EXISTS

INIT

Why?

1 Answer

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by (25.1k points)

Since the __new__ function is called when creating the instance and __init__ is called when initializing the instance and these two steps happen in this order only, so the __new__ function is always called first and __init__ function is called after that.

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