base64.urlsafe_b64encode() almost always returns strings that include the = character. this may be ok before the ? in a URL, but it's not OK after.

it would be nice if it substituted another character for =, like it does for + and /. if this is intentional, though, and you don't want to substitute for =, the documentation should probably be changed to note that it's only safe for use before the ?. (it doesn't include that caveat now.)

http://docs.python.org/lib/module-base64.html#l2h-1592