[Web-SIG] Web Site Process Bus

[Phillip J. Eby]

At 02:28 PM 6/25/2007 -0500, Ian Bicking wrote:
>Potentially a Zope-style minimal event framework would work.  Maybe 
>something like:
>
>   send_signal(signal_name, signal_data)
>   subscribe(signal_name, listener)

That was what I was wondering, too, except I was thinking it would be 
sufficient to use entry points for subscription, but only invoke the 
entry points whose modules are in sys.modules.  In other words, never 
actually import a module in order to invoke a callback.  That way, 
subscription is a natural side effect of importing the modules that 
contain the listeners.  Something like:

def send_signal(group, name, *args, **kw):
     for ep in iter_entry_points(group, name):
         if ep.module_name in sys.modules:
             ep.load()(*args, **kw)

would then be sufficient.  pkg_resources is currently proposed for 
inclusion in Python 2.6 (per PEP 365), so there wouldn't even be any 
non-stdlib requirements then.  And of course the implementation is 
trivial enough to copy into any code that wants to send signals.

Of course, if you want prioritized callbacks, you can always sort the 
ep.load() results using a priority attribute or some such.  I'm not 
sure priority makes much sense, though, due to the difficulty of 
determining a global priority scheme.