Скачать или смотреть Resolving TypeError: Subscripted Generics Cannot Be Used With Class and Instance Checks in Python

Summary: Learn how to address the `TypeError: Subscripted Generics Cannot Be Used With Class and Instance Checks` in Python, specifically in scenarios involving FastAPI.
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Resolving TypeError: Subscripted Generics Cannot Be Used With Class and Instance Checks in Python

Python programmers often encounter various types of errors, and one particularly perplexing one is the TypeError: subscripted generics cannot be used with class and instance checks. This error can surface in multiple contexts, including web frameworks like FastAPI. In this guide, we'll delve into this error, why it occurs, and how to resolve it.

Understanding the Error

The TypeError: subscripted generics cannot be used with class and instance checks error is primarily associated with type hints introduced in Python 3.5 through the typing module. Subscripted generics like List[int], Dict[str, Any], etc., enhance code readability and aid static type checks. However, these subscripted generics are not designed to work with type checks using the isinstance() and issubclass() functions.

Example Scenario

Consider this code snippet:

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Running this code will lead to the following TypeError:

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Why Does It Happen?

The root of the issue lies in Python's type system. Python's subscripting feature (e.g., List[int]) comes from the typing module for the purpose of type hinting and does not transform List[int] into a valid type for isinstance() checks. Essentially, List[int] isn't recognized as a regular class or instance type by Python's runtime.

Fixing the Error

To resolve this error, you need to adjust your type checks. Instead of checking for List[int], you should check against the base type (e.g., list) and implement additional logic if needed to verify the contents. Below are several possible fixes:

Solution 1: Check Base Type

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Solution 2: Manual Content Check

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This approach first checks that data is a list and then verifies that each element in the list is an integer.

Fixing in FastAPI

FastAPI, a modern web framework for building APIs with Python 3.6+, may also lead you to encounter this error, especially when using type hints for request bodies or query parameters.

Example in FastAPI

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In the above example, List[str] is used as a type hint for query parameters. This is perfectly valid. However, an incorrect use in type checks inside the API logic may cause the TypeError.

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

Fix in FastAPI

Instead, you should perform the type check using the base type:

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

Conclusion

Encountering the TypeError: subscripted generics cannot be used with class and instance checks can be confusing, but it's straightforward to resolve once you understand why it occurs. By adjusting your type checks to use base types and manually validating the contents when necessary, you can effectively sidestep this issue in both general Python programming and frameworks like FastAPI. Happy coding!