Скачать или смотреть How to Properly Override a Method in a Single Instance of a Python Class

Discover the right way to override methods in Python classes without affecting all instances. Learn to handle the 'self' parameter correctly for a seamless experience!
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This video is based on the question https://stackoverflow.com/q/73750464/ asked by the user 'g00dds' ( https://stackoverflow.com/u/19369393/ ) and on the answer https://stackoverflow.com/a/73750650/ provided by the user 'zalevskiaa' ( https://stackoverflow.com/u/19209687/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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The Challenge of Overriding Methods in Python Classes

When working with Python classes, developers often want the flexibility to modify methods for a specific instance without changing the behavior of the entire class. However, an attempting override can lead to complications, particularly involving the self parameter.

In this post, we discuss a common scenario:
You have a class with a method that you would like to override within just one instance of that class. The expectation is to call the original method of the class and then, after modification, call the new override successfully.

The Problem in the Code

Let's examine the following piece of code that raised some issues:

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

The output you expect should be:

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

However, running the code results in a TypeError, indicating that new_func() is missing a required positional argument: self.

Why the Error Occurs

The crux of the issue stems from how Python treats bound and unbound methods. When you assign a function directly as you did with instance.func = new_func, it loses its connection to the instance and becomes a regular function that requires a self parameter.

As such, calling instance.func() leads to a missing context for self, causing the error.

How to Properly Override a Method for a Single Instance

To achieve the desired functionality, you can either adjust the class method directly before instantiation or utilize the types.MethodType method to bind your new function correctly.

Solution 1: Change the Class Method Before Instantiation

By modifying the class itself before creating an instance, you can effectively change the behavior for new instances:

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

Solution 2: Use types.MethodType to Bind a New Method

If you prefer to change the instance's method without affecting the class, Python provides a neat trick using types.MethodType. Here's how:

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

Conclusion

In conclusion, while Python makes it simple to create and manipulate classes, overriding methods can get tricky, especially when it comes to maintaining the correct binding of the instance. By following the strategies outlined above—either modifying the class method before instantiation or using types.MethodType to bind the method correctly—you can override instance methods effectively and avoid common pitfalls.

Happy coding! If you have any further questions, feel free to ask in the comments below!