How to Type Hint an Object Wrapped in a Mock in Python

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Understanding Type Hinting with Mocks in Python

The Problem: Type Hinting Issues with Mocks

In Python, the Mock class is a powerful tool for creating mock objects in your tests. It allows you to track how an object is being used, including method calls and their parameters. However, when you try to combine Mock with the wraps feature, you might run into type hinting issues.

Consider this example:

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

In this example, the type hinting using Union[Mock, Event] is causing a type error. The code works at runtime, but mypy fails to recognize that the set method is a part of the Mock and not the Event. This can lead to frustration and confusion about how to properly annotate your mocks.

The Solution: Correct Type Hints

To address this type hinting problem, you simply need to adjust your approach. Instead of using Union, you can directly annotate spy_event as a Mock. Here’s the corrected version of how to type hint the object:

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

Why This Works

Direct Annotation: By typing spy_event directly as a Mock, you inform the type checker that this variable is indeed a mock object, regardless of what it wraps.

Avoiding Misunderstanding of Union: The Union type hint implies that the variable can be either type. In this case, you always want spy_event to behave as a Mock, so there’s no need for the Union type.

Accurate Type Checking: This approach allows mypy to accurately check the methods available to spy_event, eliminating the errors encountered with the previous code.

Conclusion

By following these best practices for type hinting, you can improve the quality of your Python applications while maintaining ease and clarity in your test cases.