How to decode base64 url in python?

Question

For Facebook fbml Apps Facebook is sending in a signed_request parameter explained here:

http://developers.facebook.com/docs/authentication/canvas

They have

Answer 1

This is the right solution. In python there is base64.b64encode but that only base64 encodes and its is different from base64 url encoding. Here is the right set to of steps to convert form base64encoded to base64urlencoded string:
1. From the resultant string, replace "/" with "_" and "+" with "-"
2. Strip the trailing "==".

Et voila! That will make it a valid string for base64 url decoding. Btw, that link in @dae.eklen 's answer above is broken now.