Question

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?

Answer 1

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.

From April 2008 post: When to use __new__ vs. __init__? on mail.python.org.

You should consider that what you are trying to do is usually done with a Factory and that’s the best way to do it. Using __new__ is not a good clean solution so please consider the usage of a factory. Here’s a good example: ActiveState Fᴀᴄᴛᴏʀʏ ᴘᴀᴛᴛᴇʀɴ Recipe.