When you want to join an absolute url path http://domain.com with a relative path /authenticate changes are you'd want do something like this

# py:2.7
absolute = "http://domain.com"
relative = "/authenticate"

joined_path = absolute + absolute
# http://domain.com/authenticate

Problems would occur if the absolute path changes to http://domain.com/ or to http://domain.com/index.html.

A better option would be to use urljoin to properly handle both of the above cases.

# py:2.7
from urlparse import urljoin

absolute = "http://domain.com"
relative = "/authenticate"

joined_path = urljoin("http://domain.com/index.html", "/authenticate")
# "http://domain.com/authenticate"