Troubleshooting XMLHttpRequest: Sending PDF Files to FastAPI Made Easy

Struggling to send PDF files to FastAPI using XMLHttpRequest? Discover the solution to common issues like CORS errors and content-type settings.
---
This video is based on the question https://stackoverflow.com/q/64857459/ asked by the user 'Xeyes' ( https://stackoverflow.com/u/10168321/ ) and on the answer https://stackoverflow.com/a/64857636/ provided by the user 'CBroe' ( https://stackoverflow.com/u/1427878/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Issue when trying to send pdf file to FastAPI through XMLHttpRequest

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Troubleshooting XMLHttpRequest: Sending PDF Files to FastAPI Made Easy

When working with web applications, developers often encounter various challenges while integrating their frontend with backend services, especially when it comes to file uploads. One such common scenario is sending a PDF file from a web client to a FastAPI backend using an XMLHttpRequest. Many developers have found themselves puzzled by errors that arise in this process. If you're facing issues with sending a PDF file to FastAPI, you're not alone! In this guide, we'll break down the problem and provide a straightforward solution.

Understanding the Problem

Imagine you have a web application locally hosted on a Python server, and you need to transmit a PDF file that a user drops onto your web UI. You've set up your FastAPI server to receive these files, but for some reason, the files never make it to your API. This could result in various errors, including issues with handling the file transfer or even CORS policy errors.

In the original use case, the developer had a simple structure where they wanted to upload a PDF file via an XMLHttpRequest as shown below:

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

However, setting the Content-Type header incorrectly led to failed uploads and TypeErrors in the FastAPI backend.

The Solution

The solution to this problem is surprisingly simple but crucial. Here’s what you need to consider:

1. Remove the Content-Type Header

In the provided code, the developer was manually setting the Content-Type header to 'multipart/form-data'. This is not required when working with FormData, as the browser automatically appends the correct Content-Type along with the necessary boundary.

Why is this important?

A multipart upload needs a boundary parameter in the Content-Type header. By setting it manually, you're omitting the boundary that the browser would normally include, which results in errors like TypeError: can't concat NoneType to bytes on the FastAPI side.

2. Updated Code Example

With the Content-Type line removed, your sendPdf function should look like this:

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

Conclusion

File uploads can be tricky, especially when dealing with FormData, CORS, and FastAPI. By removing the manual setting of the Content-Type header, you allow the browser to handle it correctly, significantly simplifying the upload process.

If you're still encountering issues with CORS, ensure that your FastAPI server has the proper CORS configurations set up to accept requests from your web client. Testing and debugging these requests will ultimately lead to more robust applications.

Got more questions about integrating FastAPI with your web client? Don't hesitate to ask! Happy coding!