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Python__init__and__new__what is the difference, many novices are not very clear about this, in order to help you solve this problem, the following small series will explain in detail for everyone, there are people who need this can learn, I hope you can gain something.

What is the__init__method?

Those of you who have written object-oriented code in Python are probably familiar with the__init__method, which is usually used to initialize an instance of a class. For example:

This is the most common use of__init__. But__init__is not actually the first method called when instantiating a class. When instantiating a class using an expression such as Persion(name, age), the first method called is actually the__new__method.

What is the__method?

The__new__method accepts the same arguments as__init__, but__init__is called after the class instance is created, and the__new__method is the method that creates the class instance.

Implementation results:

By running this code, we can see that the call to the__new__method occurred before__init__. In fact, when you instantiate a class, the specific execution logic is like this:

1. p = Person(name, age)

2. First execute the__new__method of the Person class using the name and age arguments. This__new__method returns an instance of the Person class (usually super(Persion, cls).__ new__(cls, ... ...) This way),

3. Then use this instance to call the__init__method of the class. The instance generated by__new__in the previous step is self in__init__.

So, the main difference between__init__and__new__is:

1.__ Init__is usually used to initialize a new instance and control the initialization process, such as adding properties and doing additional operations, after the class instance has been created. It is an instance-level method.

2.__ new__is usually used to control the process of generating a new instance. It is a class-level method.

But having said that, what is the most common use of__new__and when do we need__new__?

3.__

According to Python's official documentation, the__new__method is basically a way to customize the instantiation process of classes when you inherit immutable classes (such as int, str, tuple). There is also the implementation of custom metaclass.

First let's look at the first function, we can use int as an example:

If we need an integer type that is always positive, we might write code like this by integrating int.

But when we run it, it turns out it's not at all what we think it is. We still get-3. This is because for an immutable object like int, we can only customize it by overloading its__new__method.

This is the modified code:

By overloading the__new__method, we achieve the desired functionality.

Another function, about custom metaclass. In fact, when I first came into contact with__new__, it was because I needed to customize metaclass, but due to space reasons, we will talk about the relationship between metaclass and__new__in python next time.

4. Use__new__to implement a single example

In fact, once we understand the__new__method, we can use it to do other interesting things, such as implementing singleton patterns in design patterns.

Because the process generated by each instantiation of a class is controlled by__new__, we can easily implement the singleton pattern by overloading the__new__method.

Output:

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