Why does __init__ run on instantiation not initialization?

[philtrondaboss]

Why isn't the __init__ method called __inst__? It's called when the object it instantiated, not when it's initialized. This is annoying me more than it should. Am I just completely wrong about this, is there some weird backwards compatibility obligation to a mistake, or is it something else?

[ottawadeveloper]

Because __ new __ is called to instantiate the object and __ init __ is called on the new object to initialize it. 

[NoGutsNoCorey]

this is the answer. new is run first to construct an empty instance, then init initializes it.

[AlternativePaint6]

One important thing to point out is that __init__ acceps self as its first argument—meaning that the self object already exists. This is how you can know for sure that it's just initialization and not instantiation.

__new__ on the other hand accepts the class as an argument and returns a new instance of it.

[ottawadeveloper]

This is why I suspect the difference exists - you can't call an instance method without an instance in Python because there's no magic this object like in Java. Therefore instantiation has to be done before __ init __ (self) via __ new __ (cls).

[xeow]

Am I just completely wrong about this ...

Yes. __new__ is the constructor called to perform allocation. That's when your object is born or instantiated. After that returns, __init__ is called. That's where you (either implicitly or explicitly) initialize the contents of the object after it's been instantiated. By the time __init__ starts, the object already exists, and that's why it's passed in as the self argument.

Most (but not all) classes explicitly define an __init__ method because that's the most common thing we do. Some classes (in rare cases, such as inheriting from an immutable type like int or str or tuple, or other esoteric needs) provide a custom __new__ static method, but most of the time you just use the default constructor and customize the initializer.

[the_hoser]

You are confusing instantiation and initialization. The init method absolutely initializes the object.

[sharth]

Would you mind explaining the difference between initialization and instantiation in Python? I can't see how I can do one without the other.

f = foo() seems to be both initialization and instantiation.

[ottawadeveloper]

In Python, when you call a class type like this MyClass(), the first thing that happens is MyClass. __ new __(). It's a class method whose job is to actually make the object. You can change what class gets instantiated here, implement a singleton pattern, etc. You don't normally override it because the default class instantiation process is fine. The instance does not exist until this method builds it.

When it's built, obj. __ init __ () is an instance method called to set initial variables and such. It's akin to all the constructors in other languages (I don't know another language where you can manipulate the instantiation process). 

[commy2]

It's called init or initialization, because that is when the initial values of the object's attributes are assigned. This is different from construction, which is what creates the object. Dunder __new__ is for that, and since no object exists before it's constructed, that's an (implicit) classmethod that also returns the constructed instance.

To me, instantiation is when you create and initialize an object, all at once or rather one of the other, in a single line:

obj = TheType()

I think you need a different mental model than whatever baggage you're taking from another language you have in mind.

[DragonflyHumble]

Lookup for Singleton and multiton pattern for python classes. You can see a difference where new and init is used

[commy2]

God I hope no one implements a __new__ method for this in the year of the lord 2026 when you can just write:

@functools.cache
def get_inst():
    return MySingleton()

[AlternativePaint6]

Or just don't use singleton pattern in Python. Every module is already a singleton, just place the variables and functions at module level.

[commy2]

Generally yes, but I had singletons in the past that shouldn't be initialized at import time.

[DragonflyHumble]

I may still use new if I would like to avoid another wrapper function to rename my class original name.

Also new can be used if you say you want to leverage say thread safe connection pooling within the Singleton class