In Flask how can I redirect to a template and show a message after returning send_file in a view?

Question

I'm diving into Flask for the first time and I'm running into a little problem. I have a page with a form and a bunch of checkboxes. When submitting the form I'm taking the values from all the checkboxes and passing that into a script (which I already had) that basically writes a CSV file.

What I do is that upon submitting the form, the CSV file is created in the background and sent back to the user for download immediately. I got this part working by making my script create the file in memory (using StringIO) and then returning it using Flask's send_file.

What I would like is to also to give the user some feedback after he downloads the file by flashing a message to the template (you could ask why do I want to notify the user if he already downloaded the file - I just want to give him some extra information). However, after my view function returns send_file and presents a download dialog in the browser, the page isn't reloaded so the flash message doesn't get through.

I'm struggling with this: how can I return the file and also show a message to the user? I understand that each request can only have one response, so if I use my one chance with the file download I might need another strategy. Any ideas?

Here's how my "download route" looks like:

@app.route('/process', methods=["POST"])
def process():
    error = None
    if request.method == 'POST':
        # gets all checkbox values
        fields = request.form.getlist("field")

        # generates my csv file
        csv = generate_csv()

        if len(fields) != 0:
            csv = amxml2csv.xml2csv(xml, *fields)
            flash("Extraction succeeded!")
            return send_file(data, attachment_filename="newresults.csv", as_attachment=True)
        else:
            error = "No fields selected!"
    return render_template("index.html", error=error)

Answer 1

I'm having the same problem here. A halfway solution so far I found is to do

return send_from_directory() and redirect(url_for())