Скачать или смотреть How to Use __getattr__ to Retrieve List Indices in Python 3

Discover how to use Python's `__getattr__` method effectively to return the index of list elements by name in your code!
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This video is based on the question https://stackoverflow.com/q/63556188/ asked by the user 'John' ( https://stackoverflow.com/u/11839859/ ) and on the answer https://stackoverflow.com/a/63558524/ provided by the user 'John' ( https://stackoverflow.com/u/11839859/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Getting List Indices with _getattr_

If you've ever tried to access elements of a list by using _getattr_ in Python, you may have run into some issues, especially when transitioning from Python 2 to Python 3. For example, consider the following requirement: you want to be able to call an element of a list by its name, retrieving the index of that element directly as if it were an attribute of a class.

Imagine you have a list containing page identifiers, like this:

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

You want to call Page_ids.PAGE3 and receive the output 2 (since PAGE3 is the third element in the list, but has an index of 2). However, doing so might raise an AttributeError indicating that Page_ids has no such attribute. So, what are you missing? Let's dive into the solution!

The Solution: Correcting the Code for Python 3

The main problem here arises from the way Python 3 handles class attributes and metaclasses compared to Python 2.

Revised Class Structure

To resolve this issue, we need to define a metaclass that utilizes _getattr_ appropriately. Here's how it can be done:

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

Key Components of the Solution

Metaclass Definition:

We create a new metaclass called Meta that inherits from type. This is where the magic happens for handling attribute retrieval.

Using _getattr_ in the Metaclass:

The _getattr_ method is overwritten to fetch the index of an element when it is called as an attribute. If you request an attribute (like PAGE3), it checks against the values list to find the corresponding index.

Implementing the Class with Metaclass:

The Page_ids class now uses Meta as its metaclass. Inside the class, we define the values list directly.

Example Execution:

When you run print(Page_ids.PAGE3), it will correctly output 2, as desired. The metaclass handles the request for the PAGE3 attribute and returns the index from the values list.

Conclusion

Using metaclasses in Python can seem daunting at first, but they offer great flexibility and power when designing your classes. By understanding and correctly implementing _getattr_ in a metaclass, you can seamlessly retrieve indices from lists by name. This technique not only simplifies your code but also enhances readability and maintainability.

Happy coding with Python!