Why is __init__() always called after __new__()?

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.

__new__ is static class method, while __init__ is instance method. __new__ has to create the instance first, so __init__ can initialize it. Note that __init__ takes self as parameter. Until you create instance there is no self.

Now, I gather, that you're trying to implement singleton pattern in Python. There are a few ways to do that.

Also, as of Python 2.6, you can use class decorators.

def singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance

@singleton
class MyClass:
  ...