Resolving the attrs Overwriting Issue in Python Inheritance

Discover how to prevent inherited fields in Python `attrs` from sharing values across instances in your classes.
---

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: python attrs inherited field value gets overwritten

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Problem: Overwritten Inherited Field Values in Python attrs

When working with classes in Python, especially when using the attrs library, one might encounter a perplexing issue: inherited field values get overwritten across instances. This can become problematic, particularly when subclasses share a common field from a parent class. In this guide, we’ll break down the problem and explore effective solutions to ensure that instances maintain their own separate field values.

The Issue at Hand

Imagine you have a base class that defines a field, and several subclasses inherit from that base class. Each instance of the subclasses should maintain their own value for that inherited field. However, when you set a value on the inherited field for one instance, it inadvertently affects the other instance. This is due to how Python handles default mutable arguments, leading to unexpected behaviors.

Key Code Explanation

Consider the following code snippet:

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

The Solution: Using a Factory for Field Initialization

The most straightforward approach to ensure distinct instances of an inherited field is to utilize a factory method to create the field’s instance. Here’s a detailed look at how you can implement this:

Step-by-Step Instructions

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

Ensure Instance Separation: The use of a factory method ensures that each time an instance of Parent (or its subclasses) is created, a new instance of MyField is generated.

Test the Implementation: Create multiple instances of your subclasses and modify the values. Each instance will now hold its individual state without interference from one another.

Example Implementation

Here’s an idiomatic way to implement this in your classes:

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

In this example, both Child1 and Child2 now maintain their own separate instances of MyField, which means changes to one will not affect the other.

Conclusion

Using the attrs library in Python can greatly enhance the clarity and structure of your classes, but it’s crucial to handle default mutable fields correctly, especially in inheritance scenarios. By following the aforementioned factory pattern, you can maintain clean and predictable behavior across your class instances. This ensures that each instance retains its unique attributes without undesired overlaps.

By understanding and applying these techniques, you can avoid the common pitfalls associated with inherited fields in your Python applications. Happy coding!