Python Flask send_file StringIO blank files
<p>I want to process a Pandas dataframe and send it to download as a CSV without a temp file. The best way to accomplish this I've seen is to use <code>StringIO</code>. Using the code below, a file downloads with the proper name, however the file is completely blank, and no error is shown. Why doesn't the file contain data?</p>
<pre><code>@app.route('/test_download', methods = ['POST'])
def test_download():
    buffer = StringIO()
    buffer.write('Just some letters.')
    buffer.seek(0)
    return send_file(
        buffer,
        as_attachment=True,
        download_name='a_file.txt',
        mimetype='text/csv'
    )
</code></pre>