Utility Registration APIs — zope.configuration 4.0 documentation

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# Utility Registration APIs

`zope.component.` `getUtility`(interface, name='', context=None)

`zope.component.` `queryUtility`(interface, name='', default=None, context=None)

Utilities are components that simply provide an interface. They are instantiated at the time or before they are registered. Here we test the simple query interface.

Before we register any utility, there is no utility available, of course. The pure instatiation of an object does not make it a utility. If you do not specify a default, you get a ComponentLookupError.

    >>> from zope.component import getUtility
    >>> from zope.component import queryUtility
    >>> from zope.component.tests.examples import I1
    >>> getUtility(I1)
    Traceback (most recent call last):
    ...
    ComponentLookupError: \
    (<InterfaceClass zope.component.tests.examples.I1>, '')

Otherwise, you get the default:

    >>> queryUtility(I1, default='<default>')
    '<default>'

Now we declare ob to be the utility providing I1:

    >>> ob = object()
    >>> from zope.component import getGlobalSiteManager
    >>> getGlobalSiteManager().registerUtility(ob, I1)

Now the component is available:

    >>> getUtility(I1) is ob
    True
    >>> queryUtility(I1) is ob
    True

## Named Utilities

Registering a utility without a name does not mean that it is available when looking for the utility with a name:

    >>> getUtility(I1, name='foo')
    Traceback (most recent call last):
    ...
    ComponentLookupError:
    (<InterfaceClass zope.component.tests.examples.I1>, 'foo')

    >>> queryUtility(I1, name='foo', default='<default>')
    '<default>'

Registering the utility under the correct name makes it available:

    >>> ob2 = object()
    >>> getGlobalSiteManager().registerUtility(ob2, I1, name='foo')
    >>> getUtility(I1, 'foo') is ob2
    True
    >>> queryUtility(I1, 'foo') is ob2
    True

## Querying Multiple Utilities

`zope.component.` `getUtilitiesFor`(interface, context=None)

`zope.component.` `getAllUtilitiesRegisteredFor`(interface, context=None)

Sometimes it may be useful to query all utilities, both anonymous and named for a given interface. The `getUtilitiesFor()` API returns a sequence of `(name, utility)` tuples, where `name` is the empty string for the anonymous utility:

    >>> from zope.component import getUtilitiesFor
    >>> tuples = list(getUtilitiesFor(I1))
    >>> len(tuples)
    2
    >>> ('', ob) in tuples
    True
    >>> ('foo', ob2) in tuples
    True

The `getAllUtilitiesRegisteredFor()` API returns utilities that have been registered for a particular interface. Utilities providing a derived interface are also listed.

    >>> from zope.interface import implementer
    >>> from zope.component.tests.examples import Comp
    >>> from zope.component.tests.examples import I2
    >>> from zope.component.tests.examples import Ob
    >>> class I11(I1):
    ...     pass

    >>> @implementer(I11)
    ... class Ob11(Ob):
    ...     pass

    >>> ob11 = Ob11()
    >>> ob_bob = Ob()

Now we register the new utilities:

    >>> from zope.component import getGlobalSiteManager
    >>> gsm = getGlobalSiteManager()
    >>> gsm.registerUtility(ob, I1)
    >>> gsm.registerUtility(ob11, I11)
    >>> gsm.registerUtility(ob_bob, I1, name='bob')
    >>> gsm.registerUtility(Comp(2), I2)

We can now get all the utilities that provide interface I1:

    >>> from zope.component import getAllUtilitiesRegisteredFor
    >>> uts = list(getAllUtilitiesRegisteredFor(I1))
    >>> len(uts)
    4
    >>> ob in uts
    True
    >>> ob2 in uts
    True
    >>> ob_bob in uts
    True
    >>> ob11 in uts
    True

Note that getAllUtilitiesRegisteredFor() does not return the names of the utilities.

## Delegated Utility Lookup

`zope.component.` `getNextUtility`(context, interface, name='')

  Get the next available utility.

  If no utility was found, a ComponentLookupError is raised.

`zope.component.` `queryNextUtility`(context, interface, name='', default=None)

  Query for the next available utility.

  Find the next available utility providing interface and having the specified name. If no utility was found, return the specified default value.

It is common for a utility to delegate its answer to a utility providing the same interface in one of the component registry’s bases. Let’s first create a global utility:

    >>> from zope.interface import Interface
    >>> from zope.interface import implementer
    >>> class IMyUtility(Interface):
    ...     pass

    >>> from zope.component.tests.examples import ConformsToIComponentLookup
    >>> @implementer(IMyUtility)
    ... class MyUtility(ConformsToIComponentLookup):
    ...     def __init__(self, id, sm):
    ...         self.id = id
    ...         self.sitemanager = sm
    ...     def __repr__(self):
    ...         return "%s('%s')" % (self.__class__.__name__, self.id)

    >>> gutil = MyUtility('global', gsm)
    >>> gsm.registerUtility(gutil, IMyUtility, 'myutil')

Now, let’s create two registries and set up the bases hierarchy:

    >>> from zope.interface.registry import Components
    >>> sm1 = Components('sm1', bases=(gsm, ))
    >>> sm1_1 = Components('sm1_1', bases=(sm1, ))

Now we create two utilities and insert them in our folder hierarchy:

    >>> from zope.component.interfaces import IComponentLookup
    >>> util1 = MyUtility('one', sm1)
    >>> sm1.registerUtility(util1, IMyUtility, 'myutil')
    >>> IComponentLookup(util1) is sm1
    True

    >>> util1_1 = MyUtility('one-one', sm1_1)
    >>> sm1_1.registerUtility(util1_1, IMyUtility, 'myutil')
    >>> IComponentLookup(util1_1) is sm1_1
    True

Now, if we ask util1_1 for its next available utility we get the `one` utility:

    >>> from zope.component import getNextUtility
    >>> getNextUtility(util1_1, IMyUtility, 'myutil')
    MyUtility('one')

Next we ask util1 for its next utility and we should get the global version:

    >>> getNextUtility(util1, IMyUtility, 'myutil')
    MyUtility('global')

However, if we ask the global utility for the next one, an error is raised

    >>> getNextUtility(gutil, IMyUtility,
    ...                     'myutil')
    Traceback (most recent call last):
    ...
    ComponentLookupError:
    No more utilities for <InterfaceClass zope.component.tests.examples.IMyUtility>,
    'myutil' have been found.

You can also use queryNextUtility and specify a default:

    >>> from zope.component import queryNextUtility
    >>> queryNextUtility(gutil, IMyUtility, 'myutil', 'default')
    'default'

Let’s now ensure that the function also works with multiple registries. First we create another base registry:

    >>> myregistry = Components()

We now set up another utility into that registry:

    >>> custom_util = MyUtility('my_custom_util', myregistry)
    >>> myregistry.registerUtility(custom_util, IMyUtility, 'my_custom_util')

We add it as a base to the local site manager:

    >>> sm1.__bases__ = (myregistry,) + sm1.__bases__

Both the `myregistry` and global utilities should be available:

    >>> queryNextUtility(sm1, IMyUtility, 'my_custom_util')
    MyUtility('my_custom_util')
    >>> queryNextUtility(sm1, IMyUtility, 'myutil')
    MyUtility('global')

Note, if the context cannot be converted to a site manager, the default is retruned:

    >>> queryNextUtility(object(), IMyUtility, 'myutil', 'default')
    'default'

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