What can `__init__` do that `__new__` cannot?

Question

In Python, __new__ is used to initialize immutable types and __init__ typically initializes mutable types. If __init__ were removed from the language, what could no longer be done (easily)?

For example,

class A:      def __init__(self, *, x, **kwargs):         super().__init__(**kwargs)         self.x = x   class B(A):      def __init__(self, y=2, **kwargs):         super().__init__(**kwargs)         self.y = y 

Could be rewritten using __new__ like this:

class A_N:      def __new__(cls, *, x, **kwargs):         obj = super().__new__(cls, **kwargs)         obj.x = x         return obj   class B_N(A_N):      def __new__(cls, y=2, **kwargs):         obj = super().__new__(cls, **kwargs)         obj.y = y         return obj 

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Clarification for scope of question: This is not a question about how __init__ and __new__ are used or what is the difference between them. This is a question about what would happen if __init__ were removed from the language. Would anything break? Would anything become a lot harder or impossible to do?

Answer 1

Note about difference between __new__ and __init__

Before explaining missing functionality let's get back to definition of __new__ and __init__:

__new__ is the first step of instance creation. It's called first, and is responsible for returning a new instance of your class.

However, __init__ doesn't return anything; it's only responsible for initializing the instance after it's been created.

Consequences of replacing __init__ with __new__

Mainly you would lose out on flexibility. You would get a lot of semantics headaches and loose separation of initializatin and construction (by joining __new__ andinit we are to joining construction and initialization into one step...). Let's take a look on snippet below:

class A(object):     some_property = 'some_value'      def __new__(cls, *args, **kwargs):         obj = object.__new__(cls, *args, **kwargs)         obj.some_property = cls.some_property         return obj   class B(A):     some_property = 2      def __new__(cls, *args, **kwargs):         obj = super(B, cls).__new__(cls)         return obj 

Consequences of moving __init__ actions into __new__:

1. Initialize B before A: When you are using __new__ method instead of __init__ your first step of creating new instance of B is calling A.__new__ as side effect you cannot initialize B before A is initialized ( access and assign some properties to new B instance). Using of __init__ gives you such flexability.

2. Loose control on initializing order: let's imagine that you have B_N inherited from two classes (A_N1, A_N2), now you would miss controlling of order of initializing new instance of B_N(what is the order you are going to initialize instances ? it could be matter... what is weird.)

3. Properties and methods mess: you would miss access to A.some_property (cls would be equal to B while instantiating new instance of B. However directly accessing of A.some_property is possible, but my guess it's at least weird to access properties within class throught class name and not by using classmethods).

4. You cannot re-initialize an existed instance without creating new one or implementation special logic for this ( thanks to @platinhom for idea )

What can __init__ do that __new__ cannot?

There are no actions that cannot be done in __new__ and can in __init__, because actions that __init__ performs is a subset of the actions that can be performed by __new__.

An interesting moment from Python Docs, Pickling and unpickling normal class instances#object.getinitargs regarding when __init__ could be usefull:

When a pickled class instance is unpickled, its init() method is normally not invoked.

Answer 2

Everything you can do in __init__ can also be done in __new__.

Then, why use __init__?
Because you don't have to store instance in variable (obj in your example code), and later bother returning it. You can focus on what you realy want to do – initializing mutable object.