[Zope-dev] summary of discussion was: adapter vs factory Re: implementing zope.component 4.0

[Martijn Faassen]

Hi there,

I'd like to summarize the options I've seen appear in the discussion so far.

We have the following options:

1) introduce a new method, such as "instance()" or "lookup()" on 
instance. It unifies utilities with adapters. We can make it do whatever 
we want without worrying about backwards compatibility.

2) introduce several new methods that distinguish between utility and 
adapter lookup. We can make them do whatever we want without worrying 
about backwards compatibility.

3) call the interface, which unifies adapter and utility lookups. Use 
tuples for multi adaptation. We think could make this work without *too* 
much backwards compatibility issues (pending research on how prevalent 
tuple adaptation really is). In the long term we can even map out a 
deprecation strategy that can smoothly migrate us to a "multi argument" 
approach.

4) call the interface, which unifies adapter and utility lookups. Use 
multiple arguments for multi adaptation. The backwards compatibility 
obstacles are largest here as we already have the "default" argument. 
We'd need to introduce multiple "modes" to selectively upgrade.

I'm in favor of calling the interface. I'm also in favor of unifying 
adapter and utility lookup.

On the back end, I'm also in favor of allowing utility creation by 
factory (or "null adaptation") and allowing instance lookup for 
instances ("contextual utility lookup" or "adaptation to an instance"). 
  I think four ways to retrieve an object of the right interface 
(combining factory/registered instance and lookup globally/lookup for an 
instance) is a good argument *against* distinguishing between creation 
strategies or "connection to adapted object or not" in the API.

If I look up a utility I wouldn't want to care whether it happened to be 
a previously registered instance or a factory created one. If I look up 
an adapter I wouldn't care whether it happened to be a previously 
registered instance either. In fact, returning a previously registered 
instance can be very well implemented using a factory. (In fact, this 
suggests to me we should actually explore implementing instance 
registration in terms of special factories.)

Regards,

Martijn