Скачать или смотреть Understanding Why __rsub__ Subtracts in Reverse in Python's Custom Classes

A guide to understanding the behavior of the `__rsub__` method in Python's custom classes, specifically when inheriting from lists, and how to correct its functionality.
---
This video is based on the question https://stackoverflow.com/q/77264959/ asked by the user 'zaelcovsky' ( https://stackoverflow.com/u/18252875/ ) and on the answer https://stackoverflow.com/a/77265064/ provided by the user 'Sylven' ( https://stackoverflow.com/u/13277887/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why `__rsub__` subtract in reverse?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Why Does _rsub_ Subtract in Reverse?

When working with custom classes in Python, especially those that inherit from built-in types like list, you may encounter unexpected behavior when it comes to operator overloading. One common question is: Why does the _rsub_ method subtract in reverse? Let’s explore this problem in detail and discover how to resolve it.

The Problem

Imagine you have a custom list class named CustomList that allows subtraction between instances of the class and standard Python lists. You’ve implemented the _sub_ method to perform the subtraction as expected. However, when you try to use the subtraction operator with a regular list as the first operand, the operation yields a surprising result.

For example, if you perform the operation:

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

You would expect the result to be:

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

Instead, you get:

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

This surprising behavior is due to how Python handles operator overloading, and specifically, the behavior of the _rsub_ method.

Understanding the Behavior of _rsub_

What is __rsub__?

The _rsub_ method is a special method in Python that stands for "reverse subtraction." It is called when the left operand doesn't support a particular operation, in this case, subtraction, and the right operand does.

When you perform the operation list - CustomList, Python calls your CustomList's _rsub_ method. If the _rsub_ method simply returns the result of the usual subtraction in the wrong order, you end up with a reversed output:

The method you defined returns myList - L instead of the expected L - myList.

The Core Issue

Subtraction is not commutative, meaning that a - b does not equal b - a. Therefore, the way you define the _rsub_ method is crucial. Returning the result of the subtraction in reverse leads to incorrect behavior.

The Solution

To resolve this issue, you not only need to adjust your implementation of __rsub__, but you should also overload the negation operator, __neg__. Here’s how to implement the solution:

Step 1: Overloading the Negation Operator

Define the _neg_ method in your CustomList class to return a new CustomList with all values negated:

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

Step 2: Adjusting the _rsub_ Method

Modify the _rsub_ method to first negate the result of the subtraction from the standard list:

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

Complete Code Example

Here is the complete CustomList class with the adjustments made:

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

Testing the Solution

Now, testing the same operation will yield the expected result:

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

Conclusion

By understanding why _rsub_ behaves the way it does and implementing a proper solution, you can avoid common mistakes when dealing with operator overloading in your custom classes. The key takeaway is to remember that subtraction is not commutative, and properly handling negation will align outputs with your expectations. Happy coding!