Скачать или смотреть Resolving File Upload Issues in Flask with XMLHttpRequest

A guide to successfully uploading files to a Flask server using `XMLHttpRequest`. We'll explore common pitfalls and how to fix them for a smooth file handling experience.
---
This video is based on the question https://stackoverflow.com/q/69149950/ asked by the user 'Themis' ( https://stackoverflow.com/u/5813879/ ) and on the answer https://stackoverflow.com/a/69156263/ provided by the user 'Themis' ( https://stackoverflow.com/u/5813879/ ) 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: Upload files to Flask server via xhr

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.
---
Uploading Files to a Flask Server Using XHR

Uploading files can often be a tricky part of web development, especially when interfacing between JavaScript and frameworks like Flask. If you're facing challenges with file uploads not being processed correctly by your Flask server, you're not alone. In this post, we'll walk through a common scenario where form data is not being received and how to solve it efficiently.

The Problem

Many developers encounter issues when trying to upload files using Flask and JavaScript. A common symptom is that the server shows an empty ImmutableMultiDict when you print out request.form and request.files. This often indicates that the data is not being transmitted correctly from the client side.

Examining the Existing Code

Let's take a look at the provided Python and JavaScript code:

Python (Flask) Code:

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

JavaScript Code:

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

The Solution

The root of the issue lies in the manual handling of request headers. Specifically, setting the Content-Type to multipart/form-data can disrupt the way files are sent. This type of header is typically managed automatically by the XMLHttpRequest, especially when you're using FormData.

Steps to Resolve

Remove the Content-Type header:
Instead of manually specifying the Content-Type, allow the XMLHttpRequest object to set it automatically. Just remove the line:

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

Re-test your application:
After removing the troubled line, test the file upload again with a sample file (like a 200KB image). You should see that the file data is now reflected correctly in request.files.

The Moral of the Story

When working with file uploads and XMLHttpRequest, a key lesson is to trust the built-in mechanisms that handle multipart form data. By not manually setting headers that can interfere with the upload process, you allow your code to function as intended. This not only simplifies your code but also reduces the likelihood of similar issues arising in the future.

Conclusion

Handling file uploads in a Flask application can be straightforward if the correct approach is taken. By understanding the intricacies of how JavaScript and Flask interact, you can easily troubleshoot and resolve common pitfalls. Next time you encounter file upload issues, remember to check your request headers and let the browser handle the details!



Feel free to explore other Flask and JavaScript integration topics and happy coding!