Issue with send_file() and send_from_directory() not sending

[christoph-mr]

I'm following the pattern here to try and send a file from temporary storage and I'm encountering an error that I don't know how to debug. I was hoping somebody here can help.

I have a view function that redirects to a function that calls send_file() (I've tried both send_file() and send_from_directory). I'm not seeing any exceptions and everything seems to work fine, but the file is not sending. When I open up dev tools in Chrome or Edge, however i can see the file in the network panel (no error code there either -- it redirected with status code 200). And if I double-click it it will then be downloaded as an attachment. Any clue why it is not sending automatically? Not sure if it matters, but I'm using Flask==2.3.2, but I've tried with Flask==3.1.2 as well.

Here's the gist of what I'm doing:

@app.route('/some-function', methods=['GET','POST'])
def some_function():
    # some stuff happens (I define filename and do some other simple things)
    if request.method == 'POST':
        updates = request.data    # get some data  -- this works fine
        df = pandas_function(updates)    # do some stuff with that data -- this works fine
        df.to_excel(os.path.join(app.cofing['UPLOAD_FOLDER'], filename)  # write file -- this works fine 
        return redirect(url_for('download_file', filename=filename))
    return render_template('some_template.html')

@app.route('/uploads/<filename>')
def download_file(filename):
    print('i just redirected')    # this prints so it seems like it redirected successfully
    return send_from_directory(os.path.join(current_app.root_path, app.config['UPLOAD_FOLDER']), filename, as_attachment=True)

Thanks for any help you can offer!

[davidism]

You're returning text (a URL), not a redirect. Use return redirect(url_for(...)).

[christoph-mr]

Sorry, that was a typo when I was creating the post. I am calling return redirect(url_for(...)) I'll update my post.

[davidism]

I can't reproduce the issue as described with the information provided. Redirecting to a view that sends a file downloads the file. Be sure to include a minimal reproducible example when asking a question.

[christoph-mr]

It's part of a pretty large project so it's hard to disentangle these two functions from the greater project. Unfortunately, if i just provide a generic one-pager it of course works without a problem. I'll try to provide something that can be reproduced. Do you have any suggestions or suspicions as to what may be going wrong? If I had an error I could hunt that down this would be much easier. Sorry, I know that's not much to go on. I'm just not even sure where to begin looking.

[davidism]

There is nothing in Flask that would cause a redirect to prevent the download of a file being served. There is no way for the server to stop the browser from downloading a file it sends. That sounds like your browser or your frontend.

[christoph-mr]

Thanks! The issue was with some JS on the front-end. I appreciate the help!