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Twisted Web in 60 seconds: storing objects in the session
jcalderone

Welcome to the 16th installment of "Twisted Web in 60 seconds". Last time I introduced the basic APIs for interacting with Twisted Web sessions. In this installment, I'll show you how you can persist objects across requests in the session object.

As I discussed last time, instances of Session last as long as the notional session itself does. Each time Request.getSession is called, if the session for the request is still active, then the same Session instance is returned as was returned previously. Because of this, Session instances can be used to keep other objects around for as long as the session exists.

It's easier to demonstrate how this works than explain it, so here's an example:

  >>> from zope.interface import Interface, Attribute, implements
 >>> from twisted.python.components import registerAdapter
 >>> from twisted.web.server import Session
 >>> class ICounter(Interface):
 ...     value = Attribute("An int value which counts up once per page view.")
 ...
 >>> class Counter(object):
 ...     implements(ICounter)
 ...     def __init__(self, session):
 ...         self.value = 0
 ...
 >>> registerAdapter(Counter, Session, ICounter)
 >>> ses = Session(None, None)
 >>> data = ICounter(ses)
 >>> print data
 <__main__.Counter object at 0x8d535ec>
 >>> print data is ICounter(ses)
 True
 >>>

What?, I hear you say.

What's shown in this example is the interface and adaption based API which Session exposes for persisting state. There are several critical pieces interacting here:

  • ICounter is an interface which serves several purposes. Like all interfaces, it documents the API of some class of objects (in this case, just the value attribute). It also serves as a key into what is basically a dictionary within the session object: the interface is used to store or retrieve a value on the session (the Counter instance, in this case).
  • Counter is the class which actually holds the session data in this example. It implements ICounter (again, mostly for documentation purposes). It also has a value attribute, as the interface declared.
  • The registerAdapter call sets up the relationship between its three arguments so that adaption will do what we want in this case.
  • Adaption is performed by the expression ICounter(ses). This is read as adapt ses to ICounter. Because of the registerAdapter call, it is roughly equivalent to Counter(ses). However (because of certain things Session does), it also saves the Counter instance created so that it will be returned the next time this adaption is done. This is why the last statement produces True.

If you're still not clear on some of the details there, don't worry about it and just remember this: ICounter(ses) gives you an object you can persist state on. It can be as much or as little state as you want, and you can use as few or as many different Interface classes as you want on a single Session instance.

With those conceptual dependencies out of the way, it's a very short step to actually getting persistent state into a Twisted Web application. Here's an example which implements a simple counter, re-using the definitions from the example above:

  from twisted.web.resource import Resource

 class CounterResource(Resource):
     def render_GET(self, request):
         session = request.getSession()
         counter = ICounter(session)
         counter.value += 1
         return "Visit #%d for you!" % (counter.value,)

Pretty simple from this side, eh? All this does is use Request.getSession and the adaption from above, plus some integer math to give you a session-based visit counter.

Here's the complete source for an rpy script based on this example:

cache()

from zope.interface import Interface, Attribute, implements
from twisted.python.components import registerAdapter
from twisted.web.server import Session
from twisted.web.resource import Resource

class ICounter(Interface):
   value = Attribute("An int value which counts up once per page view.")

class Counter(object):
   implements(ICounter)
   def __init__(self, session):
       self.value = 0

registerAdapter(Counter, Session, ICounter)

class CounterResource(Resource):
   def render_GET(self, request):
       session = request.getSession()
       counter = ICounter(session)  
       counter.value += 1
       return "Visit #%d for you!" % (counter.value,)

resource = CounterResource()

One more thing to note is the cache() call at the top of this example. As with the previous example where this came up, this rpy script is stateful. This time, it's the ICounter definition and the registerAdapter call that need to be executed only once. If we didn't use cache, every request would define a new, different interface named ICounter. Each of these would be a different key in the session, so the counter would never get past one.

There's one more interesting thing you can do with sessions in Twisted Web right out of the box. Tune in next time to find out what.


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