Guido van Rossum, the creator of Python, wrote a web crawler as a motivating example for asyncio. You can find the code for it here:

https://github.com/aosabook/500lines/tree/master/crawler

And a detailed post about its design, co-written with A. Jesse Jiryu Davis, here:

http://aosabook.org/en/500L/a-web-crawler-with-asyncio-corou...