Martijn Faassen wrote:
> 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"
> 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.)
I am also in favor of unifying adapter and utility lookup. Or at least
creating a more normalized API.
On the syntax of the change:
I am more or less somewhere between -0 and +0 on the idea of presenting a
unified API as methods of interfaces (call or non-call). While I don't think
this idea is the worst idea in the world and it might be better than the
current global API, I don't think we should *only* do this; similar changes
should be made to the registry itself. Code that uses the global API needs to
do a registry lookup for each usage, so it's slower than code which doesn't.
If interface methods become available to unify adapter and utility lookup, the
same sorts of methods should be added to the registry itself for
performance-sensitive code. I also believe that code that locates a registry
explicitly and calls lookup methods on it is easier to understand for the
maintenance programmer than is an equivalent global API.
On the semantics of the change:
Personally I think that it's a fantasy to believe that the difference between
an object created via a factory on-demand and an object simply returned should
*never* matter to a caller. You may not want the caller to need to care, and
it may be inconvenient to take into account circumstances where the caller
needs to care. But because this is Python, the caller still often does need to
The only circumstance where a caller would *never* need to care would be when
the object returned could be guaranteed to *never ever* have any methods called
on it that weren't part of the interface defined by the interface being looked
up. If we want this, we should probably just be using Java.
I am extremely uncomfortable with any situation where:
... does not result in any factory invocation. I hope it's obvious why this is
The current global API has a modicum "kindness" for maintenance developers
because we *haven't* unified adapter and utility lookup in such a way. They
stand a shot at understanding that the result of getUtility usually has
"lifetime A" and the result of getAdapter/getMultiAdapter has "lifetime B". If
we take out this safety belt, I'd lean towards more explicitness rather than
less: let the caller call the result of the lookup.
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