Testing FastAPI File Upload with FormData

Learn how to successfully test file uploads with FastAPI using FormData for metadata. Understand the importance of using the correct request type to avoid errors during uploads.
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This video is based on the question https://stackoverflow.com/q/63614660/ asked by the user 'Nicolas' ( https://stackoverflow.com/u/13958880/ ) and on the answer https://stackoverflow.com/a/63614696/ provided by the user 'Nicolas' ( https://stackoverflow.com/u/13958880/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Testing FastAPI File Upload with FormData: A Step-by-Step Guide

Uploading files using FastAPI can often lead to issues if the request format is not set up correctly. This post will walk you through the necessary steps to test file uploads with metadata correctly using FastAPI’s FormData. We will address common pitfalls and show you how to make your uploads smooth and reliable.

The Problem

You’re trying to upload a file and its associated metadata using Python and FastAPI, but you encounter errors during testing. Specifically, you’re getting error messages that indicate required fields are missing. This is often tied to how the request is formatted, particularly when dealing with file uploads and FormData.

Here’s a quick overview of the route you have set up for file uploads:

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

Despite this setup, you still encounter errors indicating that various required fields are missing when running your unittest tests.

Understanding the Solution

The solution to your problem lies in how you’re sending your metadata along with the file upload in your unittest. Specifically, you have two options for sending data along with your request: JSON and FormData. In this case, since you are using FastAPI’s FormData to collect the upload data, you need to make sure you're sending the form data correctly.

Required Changes

Update the Request in Your Test:
Instead of using json=self.metadata, modify your request to use data=self.metadata. This change ensures that you're sending the metadata in the form data format, which matches what FastAPI is expecting. Here’s how to implement that change:

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

Why This Matters

By default, when you use json, FastAPI tries to parse the request as JSON data, which doesn't work with form fields that are expected to be sent through FormData. The use of data informs FastAPI that you are sending data in FormData format, eliminating the field required errors for the required metadata fields.

Full Example of Testing Setup

Here’s a simplified overview of the entire testing setup, integrating the discussed changes:

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

Conclusion

Testing file uploads in FastAPI requires careful attention to how you send your metadata. By switching from json=self.metadata to data=self.metadata, you align your request format with the expectations of FastAPI’s FormData handling. This simple change can save you a lot of time debugging your uploads and help ensure that your testing process goes smoothly.

Now, you have everything you need to implement file uploads with metadata in your FastAPI applications effectively!