Channels -- a question about how ProtocolTypeRouter works

[Zhiyu/Drew Li]

Hi,

Newbie here. (please bear with me if this a stupid question)

I am going through the tutorial and doc and got this question regarding how ProtocolTypeRouter works.
ProtocolTypeRouter.__init__ takes a dict "application_mapping" and append a key-value pair to it: "http': class AsgiHandler
As doc says ASGI application is created on a per scope basis, I think I vaguely understand how the "http" one works. 
Once a http scope comes in, an instance/object of class AsgiHandler is initiated, which lasts for the duration of the scope.
So it is one scope leads to one ASGI application/object

But I am confused about this one:
application = ProtocolTypeRouter({
    # (http->django views is added by default)
    'websocket': AuthMiddlewareStack(
        URLRouter(
            chat.routing.websocket_urlpatterns
        )
    ),
})
The whole value of this key-value pair AuthMiddlewareStack(URLRouter(chat.routing.websocket_urlpatterns)) seems to be a object not a class, 
so that means the application already exists before a websocket scope comes in. How does this conform to "one scope one application" rule?

Any help would be appreciated.

Thanks
Drew

[Andrew Godwin]

Hi Drew,

The return value is indeed an object, but if you look closely at it you'll see it defines a __call__ method - meaning that when it's called as an ASGI application, it then returns a further object to act inside the scope.

Andrew

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[Zhiyu (Drew) Li]

Thanks!  So that means some objects last across multiple scopes, and they can create another obj that only lasts for one specific scope?

I am still confused how the different layers of middleware wrap around the real asgi application -- a obj just lasts for one scope. 

I need to go through the code deeper.  

Thanks

Drew

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[Andrew Godwin]

I would instead think of the object that ends up there as a factory that makes the final objects that end up per-scope. It's slightly confusing, I will grant you, but in Python anything can make a new class instance, not just a class constructor, which is what's happening here.

Andrew

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