__new__ and __init__ in Python

Question

I am learning Python and so far I can tell the things below about __new__ and __init__ :

1. __new__ is for object creation
2. __init__ is for object initialization
3. __new__ is invoked before __init__ as __new__ returns a new instance and __init__ invoked afterwards to initialize inner state.
4. __new__ is good for immutable object as they cannot be changed once they are assigned. So we can return new instance which has new state.
5. We can use __new__ and __init__ for both mutable object as its inner state can be changed.

But I have another questions now.

1. When I create a new instance such as a = MyClass("hello","world") , how these arguments are passed? I mean how I should structure the class using __init__ and __new__ as they are different and both accepts arbitrary arguments besides default first argument.
2. self keyword is in terms of name can be changed to something else? But I am wondering cls is in terms of name is subject to change to something else as it is just a parameter name?

I made a little experiments as such below:

>>> class MyClass(tuple):
    def __new__(tuple):
        return [1,2,3]

and I did below:

>>> a = MyClass()
>>> a
[1, 2, 3]

Albeit I said I want to return tuple , this code works fine and returned me [1,2,3] . I knew we were passing the first parameters as the type we wanted to receive once the __new__ function is invoked. We are talking about New function right? I don't know other languages return type other than bound type?

And I did anther things as well:

>>> issubclass(MyClass,list)
False
>>> issubclass(MyClass,tuple)
True
>>> isinstance(a,MyClass)
False
>>> isinstance(a,tuple)
False
>>> isinstance(a,list)
True

I didn't do more experiment because the further wasn't bright and I decided to stop there and decided to ask StackOverflow.

The SO posts I read:

1. Python object creation
2. Python's use of __new__ and __init__?

Answer 1

how I should structure the class using __init__ and __new__ as they are different and both accepts arbitrary arguments besides default first argument.

Only rarely will you have to worry about __new__ . Usually, you'll just define __init__ and let the default __new__ pass the constructor arguments to it.

self keyword is in terms of name can be changed to something else? But I am wondering cls is in terms of name is subject to change to something else as it is just a parameter name?

Both are just parameter names with no special meaning in the language. But their use is a very strong convention in the Python community; most Pythonistas will never change the names self and cls in these contexts and will be confused when someone else does.

Note that your use of def __new__(tuple) re-binds the name tuple inside the constructor function. When actually implementing __new__ , you'll want to do it as

def __new__(cls, *args, **kwargs):
    # do allocation to get an object, say, obj
    return obj

Albeit I said I want to return tuple , this code works fine and returned me [1,2,3] .

MyClass() will have the value that __new__ returns. There's no implicit type checking in Python; it's the responsibility of the programmer to return the correct type ("we're all consenting adults here"). Being able to return a different type than requested can be useful for implementing factories: you can return a subclass of the type requested.

This also explains the issubclass / isinstance behavior you observe: the subclass relationship follows from your use of class MyClass(tuple) , the isinstance reflects that you return the "wrong" type from __new__ .

For reference, check out the requirements for __new__ in the Python Language Reference .

Edit : ok, here's an example of potentially useful use of __new__ . The class Eel keeps track of how many eels are alive in the process and refuses to allocate if this exceeds some maximum.

class Eel(object):
    MAX_EELS = 20
    n_eels = 0

    def __new__(cls, *args, **kwargs):
        if cls.n_eels == cls.MAX_EELS:
            raise HovercraftFull()

        obj = super(Eel, cls).__new__(cls)
        cls.n_eels += 1
        return obj

    def __init__(self, voltage):
        self.voltage = voltage

    def __del__(self):
        type(self).n_eels -= 1

    def electric(self):
        """Is this an electric eel?"""
        return self.voltage > 0

Mind you, there are smarter ways to accomplish this behavior.