If you're protecting a single page and need no session persistence.
class MainPage(webapp.RequestHandler):
def post(self):
if self.request.get('user') == 'admin' and self.request.get('pass') == 'soopersecure':
self.response.out.write('authorized');
else:
self.response.out.write("""
<form method="post">
<input type="text" name="user"/>
<input type="password" name="pass"/>
<input type="submit" value="login"/>
</form>""")
Otherwise you could hash the username + salt and hand it to user as a session ID in a cookie and store that session ID into the datastore. Much simpler to use Google accounts though.
http://code.google.com/appengine/docs/python/gettingstarted/usingusers.html