[Interface-dev] Proposal: define meaning of None return from an adapter factory

[Jim Fulton]

Currently, an adapter factory can return None.  This usually makes it
look as if an adapter is not defined.  For example, it would make
a call like IFoo(ob) raise a TypeError.  However, if you did IFoo(ob, 42)
and an adapter factory was found and returned None, then IFoo(ob, 42)
would return None, not 42.

I'd like to change the adapter-registry API so that a None return
from an adapter factory is defined to mean that there is no adapter.
This allows:

- preventing adaptation, when desired

- an adapter factory to determine whether adaptation is
   possible based on the state of the object being adapted.

This change would primarily cause a change to the documentation
and tests.  There would be a minor change to the querying APIs to
return a passed default value if an factory returns None.

Any objections?

Jim

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