Скачать или смотреть Understanding the mypy Error: Unsupported Operand Types for + in Python Dataclasses

Troubleshooting mypy errors related to unsupported operand types in Python dataclasses. Learn how to effectively resolve and understand the issue for cleaner code.
---
This video is based on the question https://stackoverflow.com/q/76837194/ asked by the user 'Thomas' ( https://stackoverflow.com/u/9751892/ ) and on the answer https://stackoverflow.com/a/76838877/ provided by the user 'Wombatz' ( https://stackoverflow.com/u/4759726/ ) 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: mypy error: Unsupported operand types for + ("Self" and "A") [operator] in Python dataclasses

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.
---
Resolving the mypy Error: Unsupported Operand Types for + in Python Dataclasses

When working with strict type hinting in Python, you may occasionally run into issues that cause confusion, particularly with mypy. One common error is the "Unsupported operand types for + " message, which can be frustrating for developers. In this guide, we'll explore this error in detail, using an illustrative code sample that highlights the problem and provides a clear, actionable solution.

The Problem

Consider the following scenario: you are developing a project that utilizes Python dataclasses and type hinting, and you encounter the following mypy error message:

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

This error arises when trying to add an instance of a class that uses Self as a type hint but receives an incompatible type instead.

Code Example

To better understand the issue, here's a simplified code snippet that reproduces the error:

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

In this example, the class A defines an addition operation through the _add_ method that specifically expects an operand of type Self. However, in the add_B method, we attempt to add self (an instance of A) to rhs.to_A() (which is of type A), and this triggers the error.

Why This Error Occurs

The crux of the problem lies in the relationship between Self and the class A:

Type Restriction: The _add_ method in class A specifies that it will only accept Self for its parameters. This means any subclass of A will lead to conflicts when you try to call add_B, because mypy cannot guarantee that rhs.to_A() (which returns an object of type A) is a valid operand for addition with Self, particularly in the event that subclasses of A are introduced.

Subclassing Issue: If someone were to create a subclass named A2, then the call to add_B would potentially fail if A was treated as an A2, creating ambiguity within the type system. Thus, mypy is programmed to alert you to this potential risk by raising the error.

The Solution

To resolve this issue, and allow for proper type checking without such conflicts, the fix is straightforward: modify the type hint for rhs in the _add_ method to specify the type explicitly as A rather than Self. Here's how to do this:

Updated Code Snippet

Change the parameter type in the _add_ method as follows:

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

By making this change, you clearly state that the addition operator can accept instances of type A, eliminating ambiguity for mypy.

Conclusion

Understanding type hints and the accompanying static type checks provided by mypy is crucial in maintaining robust and error-free code. By correctly defining operand types in your dataclasses, you can avoid common pitfalls associated with unsupported operand types. Remember that clarity in type definitions not only pleases the mypy but also makes the code easier to read and maintain.

With these modifications, you're well on your way to achieving clear and effective type hinting in your Python projects!