Outputting CSV Files

Generating a CSV (or PDF, etc.) report and making it available as a downloadable file is a fairly common back-end service task.

The easiest approach is to simply write CSV rows to an io.StringIO stream, and then assign its value to resp.text:

class Report:

    def on_get(self, req, resp):
        output = io.StringIO()
        writer = csv.writer(output, quoting=csv.QUOTE_NONNUMERIC)
        writer.writerow(('fruit', 'quantity'))
        writer.writerow(('apples', 13))
        writer.writerow(('oranges', 37))

        resp.content_type = 'text/csv'
        resp.downloadable_as = 'report.csv'
        resp.text = output.getvalue()

Here we set the response Content-Type to "text/csv" as recommended by RFC 4180, and assign the downloadable file name report.csv via the Content-Disposition header (see also: How can I serve a downloadable file with Falcon?).

Streaming Large CSV Files on the Fly

If generated CSV responses are expected to be very large, it might be worth streaming the CSV data as it is produced. This approach will both avoid excessive memory consumption, and reduce the viewer’s time-to-first-byte (TTFB).

In order to stream CSV rows on the fly, we will initialize the CSV writer with our own pseudo stream object. Our stream’s write() method will simply accumulate the CSV data in a list. We will then set resp.stream to a generator yielding data chunks from this list:

class Report:

    class PseudoTextStream:
        def __init__(self):

        def clear(self):
            self.result = []

        def write(self, data):

    def fibonacci_generator(self, n=1000):
        stream = self.PseudoTextStream()
        writer = csv.writer(stream, quoting=csv.QUOTE_NONNUMERIC)
        writer.writerow(('n', 'Fibonacci Fn'))

        previous = 1
        current = 0
        for i in range(n+1):
            writer.writerow((i, current))
            previous, current = current, current + previous

            yield from stream.result

    def on_get(self, req, resp):
        resp.content_type = 'text/csv'
        resp.downloadable_as = 'report.csv'
        resp.stream = self.fibonacci_generator()