How to sign amazon web service requests from the python app engine?

Question

I use Amazon web service api from within my Google app engine application. Amazon have said that they will only accept signed requests from Aug 15, 2009. While they have given

Answer 1

Pycrypto will work fine - it's supported on App Engine, though the public ciphers are implemented in Python rather than C. You also ought to be able to use one of the existing AWS libraries, now that urlfetch/httplib are supported on App Engine.

I have an app that uploads images to S3, and I've implemented the request signing myself, but mostly because I wrote it before urlfetch/httplib were available. It works just fine, however.