I've found several messages on this list discussing ways to send large files
in a HttpResponse. One can use FileWrapper, or one can use a generator and
yield chunks of the large file. What about the case when the large file is
generated at HTTP request time? In this case, it would be annoying to have
the user wait for the page to generate the large file and then stream the
file. Instead we would want a way to start the HTTP response (so that the
user gets the download dialogue), generate the large file, and then stream
the file. Let's take the following example:
def create_tarball():
> path = create_some_big_tarball()
>
> chunk = None
> fh = open(path, 'r')
> while True:
> chunk = fh.read(1024 * 128)
> if chunk == '':
> break
> yield chunk
>
> def sample_view(request):
> response = HttpResponse(create_tarball(),
> mimetype='application/x-compressed')
> response['Content-Disposition'] = "attachment;filename=mytarball.tar.gz"
>
The above example nearly accomplishes what we want, but it doesn't start the
HTTP response before the tarball is created, hence making the user wait a
long time before the download dialogue box shows up. Let's try something
like this (notice the addition of a noop yield):
def create_tarball():
yield '' # noop to send the HTTP headers
> path = create_some_big_tarball()
>
> chunk = None
> fh = open(path, 'r')
> while True:
> chunk = fh.read(1024 * 128)
> if chunk == '':
> break
> yield chunk
>
> def sample_view(request):
> response = HttpResponse(create_tarball(),
> mimetype='application/x-compressed')
> response['Content-Disposition'] = "attachment;filename=mytarball.tar.gz"
>
The issue with the above example is that the "yield ''" seems to be
ignored. HTTP headers are not sent before the tarball is created.
Similarly, "yield ' '" and "yield None" don't work, because they corrupt the
tarball (HttpResponse calls str() on the iterable items given to the
HttpResponse constructor). As a temporary solution, we're writing an empty
gzip file in the first yield. Our large tarball is gzipped, and since gzip
files can be concatenated to one and other, our hack seems to be working.
In the above example, replace the first "yield ''" with:
noop = StringIO.StringIO()
> empty = gzip.GzipFile(mode='w', fileobj=noop)
> empty.write("")
> empty.close()
> yield noop.getvalue()
>
I'm wondering if there is a better way to accomplish this? I don't quite
understand why HTTP responses are written to stdout. Possibly orthogonal to
that, it seems like, in theory, yielding an empty value in the generator
should work, because a flush is called after the HTTP headers are written.
Any ideas, either on how to solve this problem with the Django API, or on
why Django doesn't send HTTP headers on a "yield ''"?
Thanks!
Alex
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