[Twisted-Python] Components

James Y Knight foom at fuhm.net
Thu Feb 26 11:58:49 EST 2004


So I want to merge the changes from Nevow that allow lazy adapter  
registration via string arguments to registerAdapter:
registerAdapter("nevow.freeform.StringRenderer","nevow.formless.String", 
"nevow.freeform.ITypedRenderer")

However, doing this necessarily means the removal of the implicit  
super-interface feature, where twisted automatically registers  
StringRenderer for all super-interfaces of ITypedRenderer. As there is  
no way to look up the super-interfaces of a string at registerAdapter  
time (without loading up the module), this feature cannot stay.

I also doubt that's actually used by anyone. I can't think of a reason  
why I'd want to do that. The test suite doesn't even test it: all tests  
pass after removing it. There is a test for the behavior of the  
components.superInterfaces function, but not for actually registering  
adapters.

Also, the current behavior is pretty much arbitrary and wrong:
 >>>>
from twisted.python.components import *

class IFoo(Interface):
   pass

class IBaz(IFoo):
   pass

class MyIFooAdapter(Adapter):
   pass

class MyIBazAdapter(Adapter):
   pass

registerAdapter(MyIBazAdapter, str, IBaz)
registerAdapter(MyIFooAdapter, str, IFoo)
<<<<
Results in:
 >>>>
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
   File "twisted/python/components.py", line 214, in registerAdapter
     raise ValueError(
ValueError: an adapter (__main__.MyIBazAdapter) was already registered.
<<<<

However,
registerAdapter(MyIFooAdapter, str, IFoo)
registerAdapter(MyIBazAdapter, str, IBaz)
works fine.
Depending on the order of adapter registration seems quite fragile to  
me. The only proper way to handle this would be to have a concept of  
'depth' ala PyProtocols.

James





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