How to Stream Large Files

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Problem

You want to use web.py to stream large files.

Solution

This is an example of how you can use web.py to stream large files. You’ll find it DOES work as advertised, but you’re running web.py’s builtin http server, you need to make sure you add the Transfer-Encoding: chunked header for it to display properly, otherwise the browser will buffer all data before displaying it to you. This http header will be set automatically (by Apache/Nginx/…) if you run web.py app with Apache/Nginx or other web server.

You can’t mix basic string and yield returns in the same method. If you use yield, you’ll have to use yield for everything because your function becomes a generator.

Simple Example

# Simple streaming server demonstration
# Uses time.sleep to emulate a large file read
import web
import time

urls = (
    "/",    "count_holder",
    "/(.*)",  "count_down",
)

app = web.application(urls, globals())

class count_down:
    def GET(self,count):
        # These headers make it work in browsers
        web.header('Content-type','text/html')

        # Again, only set this header if you're running web.py's builtin http server.
        # You should never run web.py's builtin http server in production, get
        # yourself a Apache/Nginx server instead, it's not hard.
        web.header('Transfer-Encoding','chunked')

        yield '<h2>Prepare for Launch!</h2>'
        j = '<li>Liftoff in %s...</li>'
        yield '<ul>'
        count = int(count)
        for i in range(count,0,-1):
            out = j % i
            time.sleep(1)
            yield out
        yield '</ul>'
        time.sleep(1)
        yield '<h1>Lift off</h1>'

class count_holder:
    def GET(self):
        web.header('Content-type','text/html')
        web.header('Transfer-Encoding','chunked')
        boxes = 4
        delay = 3
        countdown = 10
        for i in range(boxes):
            output = '<iframe src="/%d" width="200" height="500"></iframe>' % (countdown - i)
            yield output
            time.sleep(delay)

if __name__ == "__main__":
    app.run()