Create and download a CSV file from a Flask view

Generate the data with csv.writer and stream the response. Use StringIO to write to an in-memory buffer rather than generating an intermediate file.

import csv
from datetime import datetime
from io import StringIO
from flask import Flask
from werkzeug.wrappers import Response

app = Flask(__name__)

# example data, this could come from wherever you are storing logs
log = [
    ('login', datetime(2015, 1, 10, 5, 30)),
    ('deposit', datetime(2015, 1, 10, 5, 35)),
    ('order', datetime(2015, 1, 10, 5, 50)),
    ('withdraw', datetime(2015, 1, 10, 6, 10)),
    ('logout', datetime(2015, 1, 10, 6, 15))
]

@app.route("https://stackoverflow.com/")
def download_log():
    def generate():
        data = StringIO()
        w = csv.writer(data)

        # write header
        w.writerow(('action', 'timestamp'))
        yield data.getvalue()
        data.seek(0)
        data.truncate(0)

        # write each log item
        for item in log:
            w.writerow((
                item[0],
                item[1].isoformat()  # format datetime as string
            ))
            yield data.getvalue()
            data.seek(0)
            data.truncate(0)

    # stream the response as the data is generated
    response = Response(generate(), mimetype="text/csv")
    # add a filename
    response.headers.set("Content-Disposition", "attachment", filename="log.csv")
    return response

If the generate function will need information from the current request, it should be decorated with stream_with_context, otherwise you will get a “working outside request context” error. Everything else remains the same.

from flask import stream_with context

@stream_with_context
def generate():
    ...