How to Successfully Upload Multiple Files with FastAPI Testing

Learn how to test uploading multiple files in FastAPI and avoid the common 422 Validation Error with our step-by-step guide.
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How to Successfully Upload Multiple Files with FastAPI Testing

If you're working with FastAPI, you might have come across the challenge of testing file uploads. A common issue developers face is encountering a 422 Validation Error when attempting to upload multiple files, despite it working perfectly in the OpenAPI interface. In this guide, we’ll walk you through how to troubleshoot this issue and successfully test uploading multiple files.

Understanding the Problem

Let’s start with the setup you might find yourself in:

You have a FastAPI router set up to handle file uploads, designed to process a list of files. The endpoint you defined looks something like this:

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

However, when you attempt to test this endpoint using FastAPI's TestClient, you encounter a 422 Validation Error. The error message often indicates that the input is not valid, specifically that a required dictionary type is missing.

Example of the Test That Fails

Your test code may look like this:

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

Despite being set up correctly, the response returns something like this:

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

The Update

Fortunately, after some troubleshooting, you might find that you're sending the files to the wrong URL or in the wrong format. Let's delve into the solution step by step.

The Solution

Correcting File Upload in Testing

To resolve the issue, ensure you format the file uploads correctly in your test. Here’s how you can set up your FastAPI application and the corresponding tests:

Updated FastAPI Application Code

Ensure your FastAPI endpoint is designed to handle a list of UploadFile objects:

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

Updated Test Code

Now, let’s correctly format the files in your test as follows:

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

Key Points to Remember

Headers: You don’t need to manually set the Content-Type header for multipart file uploads; the TestClient handles this for you.

File Format: Ensure that you are passing the files as a list of tuples where each tuple contains the field name and the file object.

Conclusion

By following the correct setup for both the FastAPI endpoint and the test cases, you can avoid the common pitfalls associated with file uploads. This guide should empower you to effectively test your FastAPI applications without encountering 422 Validation Errors.

With the right format in hand, you're now set to smoothly handle file uploads in your FastAPI projects. Happy coding!