Python Flask send_file StringIO blank files

Question

I'm using python 3.5 and flask 0.10.1 and liking it, but having a bit of trouble with send_file. I ultimately want to process a pandas dataframe (from Form data, which is unused in this example but necessary in the future) and send it to download as a csv (without a temp file). The best way to accomplish this I've seen is to us StringIO.

Here is the code I'm attempting to use:

@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,\
    attachment_filename = 'a_file.txt', mimetype = 'text/csv')

A file downloads with the proper name, however the file is completely blank.

Any ideas? Issues with encoding? Has this been answered elsewhere? Thanks!

Answer 1

The issue here is that in Python 3 you need to use StringIO with csv.write and send_file requires BytesIO, so you have to do both.

@app.route('/test_download')
def test_download():
    row = ['hello', 'world']
    proxy = io.StringIO()

    writer = csv.writer(proxy)
    writer.writerow(row)

    # Creating the byteIO object from the StringIO Object
    mem = io.BytesIO()
    mem.write(proxy.getvalue().encode('utf-8'))
    # seeking was necessary. Python 3.5.2, Flask 0.12.2
    mem.seek(0)
    proxy.close()

    return send_file(
        mem,
        as_attachment=True,
        attachment_filename='test.csv',
        mimetype='text/csv'
    )

Answer 2

I guess you should write bytes.

from io import BytesIO    

from flask import Flask, send_file


app = Flask(__name__)


@app.route('/test_download', methods=['POST'])
def test_download():
    # Use BytesIO instead of StringIO here.
    buffer = BytesIO()
    buffer.write(b'jJust some letters.')
    # Or you can encode it to bytes.
    # buffer.write('Just some letters.'.encode('utf-8'))
    buffer.seek(0)
    return send_file(buffer, as_attachment=True,
                     attachment_filename='a_file.txt',
                     mimetype='text/csv')


if __name__ == '__main__':
    app.run(debug=True)

Answer 3

if someone use python 2.7 with Flask and got the error about the module StringIO by importing it. This post can help you to solve your problem.

If you are importing String IO module, you can just change the import syntax by using this : from io import StringIO instead from StringIO import StringIO.

You can Also use from io import BytesIO if you are using image or some others ressource.

Thank you