Persisting Webapp2 Sessions on GAE

Question

I am having trouble persisting my session variables in a webapp2 session store, while running on GAE. I have created a super simple app to reproduce the issue:

class MainHandler(webapp2.RequestHandler):

    def post(self):

        session = self.session_store.get_session()

        previous_value = session.get("myvalue")
        self.response.write(previous_value)

    def get(self):

        session = self.session_store.get_session()

        previous_value = session.get("myvalue")
        self.response.write(previous_value)

        session["myvalue"] = "Hi! " + (previous_value if previous_value else "")

    # this is needed for webapp2 sessions to work
    def dispatch(self):

        # Get a session store for this request.
        self.session_store = sessions.get_store(request=self.request)

        try:
            super(MainHandler, self).dispatch()

        finally:
            # Save all sessions.
            self.session_store.save_sessions(self.response)

The premise here is that the get request sets a session variable while the post simply writes to the response object.

My understanding is that after performing the get request, the session persists myvalue. After that, if I perform a post request, then a get request again, myvalue should still be there, even though I didn't set it again in the post handler. Alas, that is not the case:

>>> cookies = None
>>> for i in range(10):
...     r = requests.get("http://localhost:11282/", cookies=cookies)
...     cookies = r.cookies
...     print r.text
... 
None
Hi! 
Hi! Hi! 
Hi! Hi! Hi! 
Hi! Hi! Hi! Hi! 
Hi! Hi! Hi! Hi! Hi! 
Hi! Hi! Hi! Hi! Hi! Hi! 
Hi! Hi! Hi! Hi! Hi! Hi! Hi! 
Hi! Hi! Hi! Hi! Hi! Hi! Hi! Hi! 
Hi! Hi! Hi! Hi! Hi! Hi! Hi! Hi! Hi! 
>>> 
>>> cookies = None
>>> for i in range(10):
...     r = requests.get("http://localhost:11282/", cookies=cookies)
...     cookies = r.cookies
...     print r.text
...     r = requests.post("http://localhost:11282/", cookies=cookies)
...     cookies = r.cookies
...     print r.text
... 
None
Hi! 
None
Hi! 
None
Hi! 
None
Hi! 
None
Hi! 
None
Hi! 
None
Hi! 
None
Hi! 
None
Hi! 
None
Hi!

Answer 1

Since I am fairly new to web technologies and standards, I seem to have missed something:

def save_session(self, response):
    if self.session is None or not self.session.modified:
        return

This is the implementation of SecureCookie. Since the cookie is not saved to the response if it is not modified, I am guessing that per standard/protocol, the backend is not responsible for sending the cookie again, as it should already be on the client. It would be great to find a reference to this.

---

This proves it (switching to requests.Session()):

>>> s = requests.Session()
>>> for i in range(5):
...     r = s.get("http://localhost:11282/")
...     print r.text
...     r = s.post("http://localhost:11282/")
...     print r.text
... 
None
POST
Hi! 
POST
Hi! Hi! 
POST
Hi! Hi! Hi! 
POST
Hi! Hi! Hi! Hi! 
POST
>>>