Streaming a CSV file in Django

Question

I am attempting to stream a csv file as an attachment download. The CSV files are getting to be 4MB in size or more, and I need a way for the user to actively download the f

Answer 1

The middleware issue has been solved as of Django 1.5 and a StreamingHttpResponse has been introduced. The following should do:

import cStringIO as StringIO
import csv

def csv_view(request):
    ...
    # Assume `rows` is an iterator or lists
    def stream():
        buffer_ = StringIO.StringIO()
        writer = csv.writer(buffer_)
        for row in rows:
            writer.writerow(row)
            buffer_.seek(0)
            data = buffer_.read()
            buffer_.seek(0)
            buffer_.truncate()
            yield data
    response = StreamingHttpResponse(
        stream(), content_type='text/csv'
    )
    disposition = "attachment; filename=file.csv"
    response['Content-Disposition'] = disposition
    return response

There's some documentation on how to output csv from Django but it doesn't take advantage of the StreamingHttpResponse so I went ahead and opened a ticket in order to track it.

Answer 2

Here's some simple code that'll stream a CSV; you can probably go from this to whatever you need to do:

import cStringIO as StringIO
import csv

def csv(request):
    def data():
        for i in xrange(10):
            csvfile = StringIO.StringIO()
            csvwriter = csv.writer(csvfile)
            csvwriter.writerow([i,"a","b","c"])
            yield csvfile.getvalue()

    response = HttpResponse(data(), mimetype="text/csv")
    response["Content-Disposition"] = "attachment; filename=test.csv"
    return response

This simply writes each row to an in-memory file, reads the row and yields it.

This version is more efficient for generating bulk data, but be sure to understand the above before using it:

import cStringIO as StringIO
import csv

def csv(request):
    csvfile = StringIO.StringIO()
    csvwriter = csv.writer(csvfile)

    def read_and_flush():
        csvfile.seek(0)
        data = csvfile.read()
        csvfile.seek(0)
        csvfile.truncate()
        return data

    def data():
        for i in xrange(10):
            csvwriter.writerow([i,"a","b","c"])
        data = read_and_flush()
        yield data

    response = HttpResponse(data(), mimetype="text/csv")
    response["Content-Disposition"] = "attachment; filename=test.csv"
    return response

Answer 3

The problem I was having was with the ConditionalGetMiddleware. I saw django-piston come up with a replacement middleware for the ConditionalGetMiddleware that allows streaming:

from django.middleware.http import ConditionalGetMiddleware

def compat_middleware_factory(klass):
    """
    Class wrapper that only executes `process_response`
    if `streaming` is not set on the `HttpResponse` object.
    Django has a bad habbit of looking at the content,
    which will prematurely exhaust the data source if we're
    using generators or buffers.
    """
    class compatwrapper(klass):
        def process_response(self, req, resp):
            if not hasattr(resp, 'streaming'):
                return klass.process_response(self, req, resp)
            return resp
    return compatwrapper

ConditionalMiddlewareCompatProxy = compat_middleware_factory(ConditionalGetMiddleware)

So then you will replace ConditionalGetMiddleware with your ConditionalMiddlewareCompatProxy middleware, and in your view (borrowed code from a clever answer to this question):

def csv_view(request):
    def data():
        for i in xrange(10):
            csvfile = StringIO.StringIO()
            csvwriter = csv.writer(csvfile)
            csvwriter.writerow([i,"a","b","c"])
            yield csvfile.getvalue()

    #create the reponse object with a csv mimetype
    response = HttpResponse(
        data(),
        mimetype='text/csv',
        )
    #Set the response as an attachment with a filename
    response['Content-Disposition'] = "attachment; filename=test.csv"
    response.streaming = True
    return response