Скачать или смотреть Understanding the __new__ Method in Python: Unraveling Its Peculiar Behavior

Discover the nuances of the `__new__` method in Python and why its behavior can sometimes seem perplexing. Learn how to manage object construction effectively!
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This video is based on the question https://stackoverflow.com/q/77914135/ asked by the user 'demetere._' ( https://stackoverflow.com/u/19125840/ ) and on the answer https://stackoverflow.com/a/77914199/ provided by the user 'chepner' ( https://stackoverflow.com/u/1126841/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Unpacking the Peculiar Behavior of Python's _new_ Method

Python's object-oriented programming (OOP) architecture is rich, but sometimes, it can lead to unexpected results, especially for those delving deeper into its functionality. One such puzzling area is the _new_ method—a crucial component in the object creation process. This blog will explore a strange behavior observed when using _new_ in different scenarios, particularly focusing on two classes, A and B. By understanding these cases, you'll gain insight into the underlying mechanics of object creation in Python.

The Problem: Unexpected Outputs

Let's illustrate the problem with the following example:

[[See Video to Reveal this Text or Code Snippet]]

When this code runs, it produces no output, even though we expect either A or B's _init_ methods to be called. Let's dig deeper into why this happens.

Understanding the First Case

In the code above, when the line a = A(1) is executed, you're essentially invoking the _call_ method of class A, which in turn calls its _new_ method. Here's a breakdown of what happens:

Calling A(1):

This translates to A.__call__(1), leading to the implementation of type.__call__(A, 1).

Inside type.__call__:

The first operation within _call_ is to trigger A.__new__(A, 1).

In the _new_ method, however, instead of passing cls (which is A), you are passing B to super().__new__(). This means that obj becomes an instance of B, not A.

Result:

Since obj is not an instance of A, obj.__init__ (which would be B.__init__) doesn't get called automatically, resulting in no output.

Making B a Subclass of A

Now, when B is defined as a subclass of A:

[[See Video to Reveal this Text or Code Snippet]]

The behavior changes dramatically. Here’s why:

Hierarchy Matter:

Now, if a = A(1) is run again, it still calls A.__new__(A, 1) but returns an instance of B, which is now also a type of A.

Init Automatically Called:

Since obj is an instance of A, its _init_ method is automatically invoked by type.__call__, leading to the output of 1, as B.__init__ is executed.

The Modified Approach: Explicit Calls

Suppose you want to ensure B.__init__ is explicitly called even when B is not a subclass of A. You'd modify your _new_ method as follows:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Modified Code

Directly Calling __init__: By invoking obj.__init__(*args, **kwargs), you ensure that B's initializer is executed regardless of the hierarchy between classes.

Control: This gives you greater control over the instantiation process, allowing you to bypass the automatic behavior of the Python OOP model.

Conclusion

Understanding the behavior of the _new_ method in Python is essential for any developer dealing with object-oriented programming in this language. While it can lead to bizarre situations—such as the one demonstrated with classes A and B—grasping how _new_ and _init_ interact will enable you to maneuver through these complexities.

From the examples above, we've learned that the relationship between the classes and their instantiation can significantly affect object creation and initialization. So next time you're working with class constructors, keep in mind the power of _new_ and the implications of class inheritance on method calls.

Happy coding!