Adapter Registry
Adapter registries provide a way to register objects that depend on one or more interface specifications and provide (perhaps indirectly) some interface. In addition, the registrations have names. (You can think of the names as qualifiers of the provided interfaces.)
The term “interface specification” refers both to interfaces and to interface declarations, such as declarations of interfaces implemented by a class.
Single Adapters
Let’s look at a simple example, using a single required specification:
>>> from zope.interface.adapter import AdapterRegistry
>>> import zope.interface
>>> class IRequireBase(zope.interface.Interface):
... pass
>>> class IProvideBase(zope.interface.Interface):
... pass
>>> class IProvideChild(IProvideBase):
... pass
>>> registry = AdapterRegistry()
We’ll register an object that depends on IRequireBase
and “provides” IProvideChild
:
>>> registry.register([IRequireBase], IProvideChild, '', 'Base->Child')
Given the registration, we can look it up again:
>>> registry.lookup([IRequireBase], IProvideChild, '')
'Base->Child'
Note that we used an integer in the example. In real applications,
one would use some objects that actually depend on or provide
interfaces. The registry doesn’t care about what gets registered, so
we’ll use integers and strings to keep the examples simple. There is
one exception. Registering a value of None
unregisters any
previously-registered value.
If an object depends on a specification, it can be looked up with a specification that extends the specification that it depends on:
>>> class IRequireChild(IRequireBase):
... pass
>>> registry.lookup([IRequireChild], IProvideChild, '')
'Base->Child'
We can use a class implementation specification to look up the object:
>>> @zope.interface.implementer(IRequireChild)
... class C2:
... pass
>>> registry.lookup([zope.interface.implementedBy(C2)], IProvideChild, '')
'Base->Child'
and it can be looked up for interfaces that its provided interface extends:
>>> registry.lookup([IRequireBase], IProvideBase, '')
'Base->Child'
>>> registry.lookup([IRequireChild], IProvideBase, '')
'Base->Child'
But if you require a specification that doesn’t extend the specification the object depends on, you won’t get anything:
>>> registry.lookup([zope.interface.Interface], IProvideBase, '')
By the way, you can pass a default value to lookup:
>>> registry.lookup([zope.interface.Interface], IProvideBase, '', 42)
42
If you try to get an interface the object doesn’t provide, you also won’t get anything:
>>> class IProvideGrandchild(IProvideChild):
... pass
>>> registry.lookup([IRequireBase], IProvideGrandchild, '')
You also won’t get anything if you use the wrong name:
>>> registry.lookup([IRequireBase], IProvideBase, 'bob')
>>> registry.register([IRequireBase], IProvideChild, 'bob', "Bob's 12")
>>> registry.lookup([IRequireBase], IProvideBase, 'bob')
"Bob's 12"
You can leave the name off when doing a lookup:
>>> registry.lookup([IRequireBase], IProvideBase)
'Base->Child'
If we register an object that provides IProvideBase
:
>>> registry.register([IRequireBase], IProvideBase, '', 'Base->Base')
then that object will be preferred over O('Base->Child')
:
>>> registry.lookup([IRequireBase], IProvideBase, '')
'Base->Base'
Also, if we register an object for IRequireChild
, then that will be preferred
when using IRequireChild
:
>>> registry.register([IRequireChild], IProvideBase, '', 'Child->Base')
>>> registry.lookup([IRequireChild], IProvideBase, '')
'Child->Base'
Finding out what, if anything, is registered
We can ask if there is an adapter registered for a collection of interfaces. This is different than lookup, because it looks for an exact match:
>>> print(registry.registered([IRequireBase], IProvideBase))
Base->Base
>>> print(registry.registered([IRequireBase], IProvideChild))
Base->Child
>>> print(registry.registered([IRequireBase], IProvideChild, 'bob'))
Bob's 12
>>> print(registry.registered([IRequireChild], IProvideBase))
Child->Base
>>> print(registry.registered([IRequireChild], IProvideChild))
None
In the last example, None
was returned because nothing was registered
exactly for the given interfaces.
lookup1
Lookup of single adapters is common enough that there is a specialized version of lookup that takes a single required interface:
>>> registry.lookup1(IRequireChild, IProvideBase, '')
'Child->Base'
>>> registry.lookup1(IRequireChild, IProvideBase)
'Child->Base'
Actual Adaptation
The adapter registry is intended to support adaptation, where one object that implements an interface is adapted to another object that supports a different interface. The adapter registry supports the computation of adapters. In this case, we have to register adapter factories:
>>> class IR(zope.interface.Interface):
... pass
>>> @zope.interface.implementer(IR)
... class X(object):
... pass
>>> @zope.interface.implementer(IProvideBase)
... class Y(object):
... def __init__(self, context):
... self.context = context
>>> registry.register([IR], IProvideBase, '', Y)
In this case, we registered a class as the factory. Now we can call
queryAdapter
to get the adapted object:
>>> x = X()
>>> y = registry.queryAdapter(x, IProvideBase)
>>> y.__class__.__name__
'Y'
>>> y.context is x
True
We can register and lookup by name too:
>>> class Y2(Y):
... pass
>>> registry.register([IR], IProvideBase, 'bob', Y2)
>>> y = registry.queryAdapter(x, IProvideBase, 'bob')
>>> y.__class__.__name__
'Y2'
>>> y.context is x
True
Passing super
objects works as expected to find less specific adapters:
>>> class IDerived(IR):
... pass
>>> @zope.interface.implementer(IDerived)
... class Derived(X):
... pass
>>> class DerivedAdapter(Y):
... def query_next(self):
... return registry.queryAdapter(
... super(type(self.context), self.context),
... IProvideBase)
>>> registry.register([IDerived], IProvideBase, '', DerivedAdapter)
>>> derived = Derived()
>>> adapter = registry.queryAdapter(derived, IProvideBase)
>>> adapter.__class__.__name__
'DerivedAdapter'
>>> adapter = adapter.query_next()
>>> adapter.__class__.__name__
'Y'
When the adapter factory produces None
, then this is treated as if no
adapter has been found. This allows us to prevent adaptation (when desired)
and let the adapter factory determine whether adaptation is possible based on
the state of the object being adapted:
>>> def factory(context):
... if context.name == 'object':
... return 'adapter'
... return None
>>> @zope.interface.implementer(IR)
... class Object(object):
... name = 'object'
>>> registry.register([IR], IProvideBase, 'conditional', factory)
>>> obj = Object()
>>> registry.queryAdapter(obj, IProvideBase, 'conditional')
'adapter'
>>> obj.name = 'no object'
>>> registry.queryAdapter(obj, IProvideBase, 'conditional') is None
True
>>> registry.queryAdapter(obj, IProvideBase, 'conditional', 'default')
'default'
An alternate method that provides the same function as queryAdapter()
is
adapter_hook()
:
>>> y = registry.adapter_hook(IProvideBase, x)
>>> y.__class__.__name__
'Y'
>>> y.context is x
True
>>> y = registry.adapter_hook(IProvideBase, x, 'bob')
>>> y.__class__.__name__
'Y2'
>>> y.context is x
True
The adapter_hook()
simply switches the order of the object and
interface arguments. It is used to hook into the interface call
mechanism.
Default Adapters
Sometimes, you want to provide an adapter that will adapt anything.
For that, provide None
as the required interface:
>>> registry.register([None], IProvideBase, '', 1)
then we can use that adapter for interfaces we don’t have specific adapters for:
>>> class IQ(zope.interface.Interface):
... pass
>>> registry.lookup([IQ], IProvideBase, '')
1
Of course, specific adapters are still used when applicable:
>>> registry.lookup([IRequireChild], IProvideBase, '')
'Child->Base'
Class adapters
You can register adapters for class declarations, which is almost the same as registering them for a class:
>>> registry.register([zope.interface.implementedBy(C2)], IProvideBase, '', 'C21')
>>> registry.lookup([zope.interface.implementedBy(C2)], IProvideBase, '')
'C21'
Dict adapters
At some point it was impossible to register dictionary-based adapters due a bug. Let’s make sure this works now:
>>> adapter = {}
>>> registry.register((), IQ, '', adapter)
>>> registry.lookup((), IQ, '') is adapter
True
Unregistering
You can unregister by registering None
, rather than an object:
>>> registry.register([zope.interface.implementedBy(C2)], IProvideBase, '', None)
>>> registry.lookup([zope.interface.implementedBy(C2)], IProvideBase, '')
'Child->Base'
Of course, this means that None
can’t be registered. This is an
exception to the statement, made earlier, that the registry doesn’t
care what gets registered.
Multi-adapters
You can adapt multiple specifications:
>>> registry.register([IRequireBase, IQ], IProvideChild, '', '1q2')
>>> registry.lookup([IRequireBase, IQ], IProvideChild, '')
'1q2'
>>> registry.lookup([IRequireChild, IQ], IProvideBase, '')
'1q2'
>>> class IS(zope.interface.Interface):
... pass
>>> registry.lookup([IRequireChild, IS], IProvideBase, '')
>>> class IQ2(IQ):
... pass
>>> registry.lookup([IRequireChild, IQ2], IProvideBase, '')
'1q2'
>>> registry.register([IRequireBase, IQ2], IProvideChild, '', '(Base,Q2)->Child')
>>> registry.lookup([IRequireChild, IQ2], IProvideBase, '')
'(Base,Q2)->Child'
Multi-adaptation
You can adapt multiple objects:
>>> @zope.interface.implementer(IQ)
... class Q:
... pass
As with single adapters, we register a factory, which is often a class:
>>> class IM(zope.interface.Interface):
... pass
>>> @zope.interface.implementer(IM)
... class M:
... def __init__(self, x, q):
... self.x, self.q = x, q
>>> registry.register([IR, IQ], IM, '', M)
And then we can call queryMultiAdapter
to compute an adapter:
>>> q = Q()
>>> m = registry.queryMultiAdapter((x, q), IM)
>>> m.__class__.__name__
'M'
>>> m.x is x and m.q is q
True
and, of course, we can use names:
>>> class M2(M):
... pass
>>> registry.register([IR, IQ], IM, 'bob', M2)
>>> m = registry.queryMultiAdapter((x, q), IM, 'bob')
>>> m.__class__.__name__
'M2'
>>> m.x is x and m.q is q
True
Default Adapters
As with single adapters, you can define default adapters by specifying
None
for the first specification:
>>> registry.register([None, IQ], IProvideChild, '', '(None,Q)->Child')
>>> registry.lookup([IS, IQ], IProvideChild, '')
'(None,Q)->Child'
Null Adapters
You can also adapt no specification:
>>> registry.register([], IProvideChild, '', '[]->Child')
>>> registry.lookup([], IProvideChild, '')
'[]->Child'
>>> registry.lookup([], IProvideBase, '')
'[]->Child'
Listing named adapters
Adapters are named. Sometimes, it’s useful to get all of the named adapters for given interfaces:
>>> adapters = list(registry.lookupAll([IRequireBase], IProvideBase))
>>> adapters.sort()
>>> assert adapters == [(u'', 'Base->Base'), (u'bob', "Bob's 12")]
This works for multi-adapters too:
>>> registry.register([IRequireBase, IQ2], IProvideChild, 'bob', '(Base,Q2)->Child for bob')
>>> adapters = list(registry.lookupAll([IRequireChild, IQ2], IProvideBase))
>>> adapters.sort()
>>> assert adapters == [(u'', '(Base,Q2)->Child'), (u'bob', '(Base,Q2)->Child for bob')]
And even null adapters:
>>> registry.register([], IProvideChild, 'bob', 3)
>>> adapters = list(registry.lookupAll([], IProvideBase))
>>> adapters.sort()
>>> assert adapters == [(u'', '[]->Child'), (u'bob', 3)]
Subscriptions
Normally, we want to look up an object that most closely matches a specification. Sometimes, we want to get all of the objects that match some specification. We use subscriptions for this. We subscribe objects against specifications and then later find all of the subscribed objects:
>>> registry.subscribe([IRequireBase], IProvideChild, 'Base->Child (1)')
>>> registry.subscriptions([IRequireBase], IProvideChild)
['Base->Child (1)']
Note that, unlike regular adapters, subscriptions are unnamed.
You can have multiple subscribers for the same specification:
>>> registry.subscribe([IRequireBase], IProvideChild, 'Base->Child (2)')
>>> registry.subscriptions([IRequireBase], IProvideChild)
['Base->Child (1)', 'Base->Child (2)']
If subscribers are registered for the same required interfaces, they are returned in the order of definition.
You can register subscribers for all specifications using None
:
>>> registry.subscribe([None], IProvideBase, 'None->Base')
>>> registry.subscriptions([IRequireChild], IProvideBase)
['None->Base', 'Base->Child (1)', 'Base->Child (2)']
Note that the new subscriber is returned first.
Subscribers defined for less specific required interfaces are returned before subscribers for more specific interfaces:
>>> class IRequireGrandchild(IRequireChild):
... pass
>>> registry.subscribe([IRequireChild], IProvideBase, 'Child->Base')
>>> registry.subscribe([IRequireGrandchild], IProvideBase, 'Grandchild->Base')
>>> registry.subscriptions([IRequireGrandchild], IProvideBase)
['None->Base', 'Base->Child (1)', 'Base->Child (2)', 'Child->Base', 'Grandchild->Base']
Subscriptions may be combined over multiple compatible specifications:
>>> registry.subscriptions([IRequireChild], IProvideBase)
['None->Base', 'Base->Child (1)', 'Base->Child (2)', 'Child->Base']
>>> registry.subscribe([IRequireBase], IProvideBase, 'Base->Base')
>>> registry.subscriptions([IRequireChild], IProvideBase)
['None->Base', 'Base->Child (1)', 'Base->Child (2)', 'Base->Base', 'Child->Base']
>>> registry.subscribe([IRequireChild], IProvideChild, 'Child->Child')
>>> registry.subscriptions([IRequireChild], IProvideBase)
['None->Base', 'Base->Child (1)', 'Base->Child (2)', 'Base->Base', 'Child->Child', 'Child->Base']
>>> registry.subscriptions([IRequireChild], IProvideChild)
['Base->Child (1)', 'Base->Child (2)', 'Child->Child']
Subscriptions can be on multiple specifications:
>>> registry.subscribe([IRequireBase, IQ], IProvideChild, '(Base,Q)->Child')
>>> registry.subscriptions([IRequireBase, IQ], IProvideChild)
['(Base,Q)->Child']
As with single subscriptions and non-subscription adapters, you can
specify None
for the first required interface, to specify a default:
>>> registry.subscribe([None, IQ], IProvideChild, '(None,Q)->Child')
>>> registry.subscriptions([IS, IQ], IProvideChild)
['(None,Q)->Child']
>>> registry.subscriptions([IRequireBase, IQ], IProvideChild)
['(None,Q)->Child', '(Base,Q)->Child']
You can have subscriptions that are independent of any specifications:
>>> list(registry.subscriptions([], IProvideBase))
[]
>>> registry.subscribe([], IProvideChild, 'sub2')
>>> registry.subscriptions([], IProvideBase)
['sub2']
>>> registry.subscribe([], IProvideBase, 'sub1')
>>> registry.subscriptions([], IProvideBase)
['sub2', 'sub1']
>>> registry.subscriptions([], IProvideChild)
['sub2']
Unregistering subscribers
We can unregister subscribers. When unregistering a subscriber, we can unregister a specific subscriber:
>>> registry.unsubscribe([IRequireBase], IProvideBase, 'Base->Base')
>>> registry.subscriptions([IRequireBase], IProvideBase)
['None->Base', 'Base->Child (1)', 'Base->Child (2)']
If we don’t specify a value, then all subscribers matching the given interfaces will be unsubscribed:
>>> registry.unsubscribe([IRequireBase], IProvideChild)
>>> registry.subscriptions([IRequireBase], IProvideBase)
['None->Base']
Subscription adapters
We normally register adapter factories, which then allow us to compute adapters, but with subscriptions, we get multiple adapters. Here’s an example of multiple-object subscribers:
>>> registry.subscribe([IR, IQ], IM, M)
>>> registry.subscribe([IR, IQ], IM, M2)
>>> subscribers = registry.subscribers((x, q), IM)
>>> len(subscribers)
2
>>> class_names = [s.__class__.__name__ for s in subscribers]
>>> class_names.sort()
>>> class_names
['M', 'M2']
>>> [(s.x is x and s.q is q) for s in subscribers]
[True, True]
Adapter factory subscribers can’t return None
values:
>>> def M3(x, y):
... return None
>>> registry.subscribe([IR, IQ], IM, M3)
>>> subscribers = registry.subscribers((x, q), IM)
>>> len(subscribers)
2
Handlers
A handler is a subscriber factory that doesn’t produce any normal
output. It returns None
. A handler is unlike adapters in that it does
all of its work when the factory is called.
To register a handler, simply provide None
as the provided interface:
>>> def handler(event):
... print('handler', event)
>>> registry.subscribe([IRequireBase], None, handler)
>>> registry.subscriptions([IRequireBase], None) == [handler]
True
Components
A zope.interface.registry.Components
object implements the
zope.interface.interfaces.IComponents
interface. This
interface uses multiple adapter registries to implement multiple
higher-level concerns (utilities, adapters and handlers), while also
providing event notifications and query capabilities.