Скачать или смотреть Understanding Why mypy Doesn't Detect Bad TypedDict Usage in Functions

Discover how to ensure `mypy` effectively checks TypedDict usage in Python code. Learn why type hints matter and how to fix common issues in your functions.
---
This video is based on the question https://stackoverflow.com/q/62509552/ asked by the user 'zmbq' ( https://stackoverflow.com/u/871910/ ) and on the answer https://stackoverflow.com/a/62522712/ provided by the user 'hoefling' ( https://stackoverflow.com/u/2650249/ ) 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: Why doesn't `mypy` detect bad TypedDict usage inside a function

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.
---
Understanding Why mypy Doesn't Detect Bad TypedDict Usage in Functions

When working with Python's type hints and structures like TypedDict, developers often rely on tools like mypy to catch potential issues in their code. However, sometimes mypy fails to flag certain problems that seem obvious, particularly when working within function scopes. This guide tackles a specific scenario: why doesn't mypy catch bad TypedDict usage inside a function?

The Problem: Undetected Bad Usage

Consider the following code snippet that uses TypedDict:

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

In this code, we have a TypedDict named P, which specifies that it should only have an integer field x. After calling return_p() and adding an unrelated key 'abc', mypy correctly raises an error on that line of code. However, within the test() function, the same issue does not throw an error when a similar key is added to p. Why does this happen?

Running mypy

Environment: This issue arose during testing on Windows 10 with Python 3.8 and mypy version 0.781. It was also replicated in Python 3.7 when importing TypedDict from typing_extensions.

The Solution: Use Type Hints

The reason behind mypy not detecting the error in the test() function lies in the absence of type hints for the function itself.

Importance of Type Hints

Type hints provide essential information to static type checkers like mypy. When a function lacks type annotations, mypy may not analyze its contents as thoroughly, which can lead to missed errors, as seen in our example.

Adding Type Hints

To make mypy aware of what's going on inside the test() function, we can add explicit type hints to its signature. Here’s how to do it:

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

Benefits of this Approach

Error Detection: With the type hint in place, mypy can now analyze the function properly and will raise an error for the line p['abc'] = 2 just like it did in the global scope.

Improved Code Clarity: By adding type annotations, the intent of the code becomes clearer, making it easier for other developers (or future you) to understand the expected data types and structures.

Conclusion

In summary, when using mypy with TypedDict, errors can sometimes be overlooked in function scopes if type hints are absent. Always remember to annotate your functions with the appropriate type hints to leverage the full capabilities of static type checking. This small change not only enhances the robustness of your code but also helps you and your team catch potential issues early in the development process.

By paying close attention to typing conventions, you can ensure that your code is reliable, maintainable, and free from unnoticed bugs.