I recently got asked to me what did
__init__ dunder method did in Python. And I was like, “it is used to initialize variables inside a class”. And just after that, the follow-up question was, “then what is
__new__ used for. And I was completely blank and was not able to answer that.
I was not able to answer that question because there are not many tutorials out there that talk about
__new__. I didn’t want to happen this with you. And that is why I came up with this blog post for you.
Let’s start with similarities.
- Both of them are called/invoked during the creation of the instance.
Let’s start with differences.
|1||Called before init||Called after new|
|2||Accepts a type as the first argument||Accepts an instance as the first argument|
|3||Is supposed to return an instance of the type received||Is not supposed to return anything|
|4||Used to control instance creation||Used to initialize instance variables|
Talking about the first point.
__new__ is called when the instance is first created. This happens before the initialization of the class.
By the way, did you note that the first argument to
__init__ is always
self? This self is the instance of the class.
self is what
Coming to the third point,
__new__ is supposed to return an instance of the class. Note that if
__new__ does not returns anything,
__init__ is not called.
Which one of them is a constructor?
If you are coming from another language, you might be surprised that there are two similar things doing the same kind of work. Most languages you might have worked with would have something called a constructor.
In Python, that concept is broken down into constructor and initializer. And you might have guessed,
__new__ is the constructor and
__init__ is the initializer.
Please note that
__new__ is implicit. Meaning that if you don’t actually need to modify the creation of an instance of the class, you don’t need to have a
One more thing I’d like to add is… instance variables are local to an instance. So anything you are doing in init is local to that instance only. But anything you are doing in new will be affecting anything created for that type.
Execution flow with an example
I am going to add some code to make this more engaging. Consider this example:
This is the simplest example of both
__init__ in action. If you save the above code in a file and run it, you’d see something like this:
$ python3 in_it.py __new__ called __init__ called
As you can see, the new method is called first and then execution is passed to the init method.
Use case for
One of the best use cases I can take an example of is when creating a Singleton. As we know, Singleton ensures a class only has one instance and provides a global point of access to it.
Some of the places I have seen singleton in action are in game programming where there is only one instance of the player. Another place, if you have used frontend libraries like Vuex (or Redux), there is only one global instance of the store. Doesn’t matter how many instances you create, you’ll end up having only one.
Let’s see how to achieve similar behavior in Python:
$ python3 singleton.py creating... <__main__.Singleton object at 0x7f943301d350> <__main__.Singleton object at 0x7f943301d350>
As you can see, creating… is printed only once. they both point to the same memory location. In my case, it’s
You might be guessing that can’t we do the same thing with
__init__? No! That’s because
__init__ does not return anything. We’ll see in the next section what
__init__ is well suited for.
But first, I wanted to show you another use case of new.
$ python3 newexample.py Initializing 4-legged animal Initializing 2-legged animal
I am by no means a zoologist. But you get my point here. You can use
__new__ to conditionally create an instance from a class.
Use case for
As we have already seen previously.
init is there to initialize an instance variable. These instance variables can later be used in different methods of the instance.
I have extensively used
__init__ when I used to work with Qt framework. Qt is a framework for desktop-based UI development. When initializing UI objects, you can set how wide or long the window could be. You can also read preferences from a file and apply that during the initialization phase of an application. Setting the window title could be another example.
Here I’ll demonstrate one such example:
The above example is by no means a complete example, but when set up correctly, it will show a window similar to this.
- In most cases, you don’t need
__new__is called before
__new__returns a instance of class.
__init__receives the instances of the class returned by
__init__to initilize value.
The same concept can be used to answer the question of abstraction vs encapsulation.
That is all I know about
__new__. If you have something in your mind that I missed. Please let me know.
I’ll also list some references I look to write this blog post so that you can really the real source of information.