How can I do this in Python?

[Jonathan Gardner]

I don't know what engine you are using. Basically all you have to do is
render the login form at the url of the page that needs the login.
You're going to have to hook in the authentication code before you
render the page normally. For instance, this is a common way I've been
doing it in various engines I use:

(1) Check to see if the user is logging in by looking at the
parameters. If they login successfully, record that in the session and
continue as normal.

(2) Normal operation being... check to see if they are logged in
according to their session. (The code in (1) will set up their session
to be logged in if they just logged in.) If they aren't and they need
to be, show the login page. Otherwise, show the regular content.