Resolving ValidationError in FastAPI: How to Return an Array of Objects Efficiently

Learn how to solve the `ValidationError` in FastAPI when returning an array of objects by employing best practices and guidelines for structuring your response model effectively.
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Resolving ValidationError in FastAPI: How to Return an Array of Objects Efficiently

FastAPI is a fantastic web framework for building APIs with Python, providing excellent speed and ease of use. However, you might encounter challenges when working with data responses, especially when trying to return arrays of objects. A common issue developers face is the ValidationError that arises from how FastAPI expects the data structure to be set up. In this post, we will dive deep into how to properly return an array of objects in FastAPI by exploring an example scenario and the different solutions available.

The Problem: Encountering ValidationError

Understanding the Error

When attempting to return a list of objects in FastAPI, you might receive an error message like this:

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

This typically happens when FastAPI fails to recognize that you are returning a list. In the given example, an endpoint was set up to return an array of InnerObject instances, but the implementation resulted in a validation error. Let's examine a simplified version of the code to illustrate the issue:

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

What Went Wrong?

The primary mistake in this code was initializing objects with [InnerObject], which does not create a list of object instances. The correct approach is to start with an empty list (objects = []) and then append actual instances of InnerObject. Let's explore the solutions.

Solutions to Return Array of Objects

Upon encountering the validation issue, you have several options to effectively return an array of objects. Here are three viable methods:

Option 1: Create Objects Explicitly

Instead of trying to return a direct list, explicitly create the OuterObject that encapsulates the list of InnerObject instances.

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

In this solution, we create the OuterObject using the list of InnerObject instances, allowing FastAPI to handle the response properly.

Option 2: Change Response Model to List

If you're only concerned about returning a list of InnerObject, you could modify the response_model for your endpoint.

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

Here, we simply state that the endpoint returns a list and return the list directly. FastAPI will handle serialization accordingly.

Option 3: Change the Definition of OuterObject

Alternatively, you could modify the OuterObject definition to inherit from List[InnerObject]. This approach can be effective if you want OuterObject to fundamentally represent a list.

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

With this adjustment, you would need to return it similar to Option 2, explicitly passing instances of InnerObject when returning data.

Conclusion

Working with arrays of objects in FastAPI does come with its set of challenges, especially concerning validation errors. Fortunately, by understanding the structure of your response model and wisely choosing how to incorporate those models, you can successfully return an array of objects without encountering validation issues.

By utilizing any of the solutions outlined above, you can ensure that your FastAPI endpoints work seamlessly, whether you opt for explicit object creation, direct lists, or adjusted class definitions.

Happy coding in FastAPI! If you have any further questions or issues, feel free to share your thoughts below!