How to Fix pydantic.error_wrappers.ValidationError in FastAPI When Using Custom Decorators

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Understanding the Problem

Imagine you have defined a couple of Pydantic models as schemas in your FastAPI application. These schemas are essential for defining the structure of the data that your API will work with. Below is a simplified version of what you might have:

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Next, you might have created a decorator for error handling across your endpoints. Here's how you might have structured your decorator:

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Finally, you have an endpoint that utilizes this decorator:

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The problem arises when you invoke this endpoint, and you receive an error message indicating a validation error with your RunsResponse model—specifically, that the response must be a valid dictionary.

Identifying the Cause of the Error

The crux of the problem lies in the way the handle_request decorator handles the asynchronous function. When you call func(*args, **kwargs) without await, it doesn’t execute the asynchronous function properly. As a result, FastAPI cannot validate the response returned, leading to the ValidationError message:

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The Solution

The solution to this issue is simple: you need to ensure that the asynchronous function inside the decorator is awaited properly. Here's the necessary adjustment to your handle_request decorator:

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By adding await before calling func(*args, **kwargs), you ensure that the asynchronous function executes as expected, allowing FastAPI to handle the result correctly.

Conclusion

Now, you can proceed with confidence in your FastAPI development, knowing how to handle errors effectively and keep your API responses validated. Happy coding!