The Zope 3 Component Architecture (Socket Example)

The component architecture provides an application framework that provides its functionality through loosely-connected components. A component can be any Python object and has a particular purpose associated with it. Thus, in a component-based applications you have many small component in contrast to classical object-oriented development, where you have a few big objects.

Components communicate via specific APIs, which are formally defined by interfaces, which are provided by the zope.interface package. Interfaces describe the methods and properties that a component is expected to provide. They are also used as a primary mean to provide developer-level documentation for the components. For more details about interfaces see zope/interface/README.txt.

The two main types of components are adapters and utilities. They will be discussed in detail later in this document. Both component types are managed by the site manager, with which you can register and access these components. However, most of the site manager’s functionality is hidden behind the component architecture’s public API, which is documented in IComponentArchitecture.

Adapters

Adapters are a well-established pattern. An adapter uses an object providing one interface to produce an object that provides another interface. Here an example: Imagine that you purchased an electric shaver in the US, and thus you require the US socket type. You are now traveling in Germany, where another socket style is used. You will need a device, an adapter, that converts from the German to the US socket style.

The functionality of adapters is actually natively provided by the zope.interface package and is thus well documented there. The human.txt file provides a gentle introduction to adapters, whereby adapter.txt is aimed at providing a comprehensive insight into adapters, but is too abstract for many as an initial read. Thus, we will only explain adapters in the context of the component architecture’s API.

So let’s say that we have a German socket:

>>> from zope.interface import Interface, implements

>>> class IGermanSocket(Interface):
...     pass

>>> class Socket(object):
...     def __repr__(self):
...         return '<instance of %s>' %self.__class__.__name__

>>> class GermanSocket(Socket):
...     """German wall socket."""
...     implements(IGermanSocket)

and we want to convert it to an US socket

>>> class IUSSocket(Interface):
...     pass

so that our shaver can be used in Germany. So we go to a German electronics store to look for an adapter that we can plug in the wall:

>>> class GermanToUSSocketAdapter(Socket):
...     implements(IUSSocket)
...     __used_for__ = IGermanSocket
...
...     def __init__(self, socket):
...         self.context = socket

Note that I could have called the passed in socket any way I like, but context is the standard name accepted.

Single Adapters

Before we can use the adapter, we have to buy it and make it part of our inventory. In the component architecture we do this by registering the adapter with the framework, more specifically with the global site manager:

>>> import zope.component
>>> gsm = zope.component.getGlobalSiteManager()
>>> gsm.registerAdapter(GermanToUSSocketAdapter, (IGermanSocket,), IUSSocket)

zope.component is the component architecture API that is being presented by this file. You registered an adapter from IGermanSocket to IUSSocket having no name (thus the empty string).

Anyways, you finally get back to your hotel room and shave, since you have not been able to shave in the plane. In the bathroom you discover a socket:

>>> bathroomDE = GermanSocket()
>>> IGermanSocket.providedBy(bathroomDE)
True

You now insert the adapter in the German socket

>>> bathroomUS = zope.component.getAdapter(bathroomDE, IUSSocket, '')

so that the socket now provides the US version:

>>> IUSSocket.providedBy(bathroomUS)
True

Now you can insert your shaver and get on with your day.

After a week you travel for a couple of days to the Prague and you notice that the Czech have yet another socket type:

>>> class ICzechSocket(Interface):
...     pass

>>> class CzechSocket(Socket):
...     implements(ICzechSocket)

>>> czech = CzechSocket()

You try to find an adapter for your shaver in your bag, but you fail, since you do not have one:

>>> zope.component.getAdapter(czech, IUSSocket, '') \
... 
Traceback (most recent call last):
...
ComponentLookupError: (<instance of CzechSocket>,
                        <InterfaceClass __builtin__.IUSSocket>,
                        '')

or the more graceful way:

>>> marker = object()
>>> socket = zope.component.queryAdapter(czech, IUSSocket, '', marker)
>>> socket is marker
True

In the component architecture API any get* method will fail with a specific exception, if a query failed, whereby methods starting with query* will always return a default value after a failure.

Named Adapters

You are finally back in Germany. You also brought your DVD player and a couple DVDs with you, which you would like to watch. Your shaver was able to convert automatically from 110 volts to 240 volts, but your DVD player cannot. So you have to buy another adapter that also handles converting the voltage and the frequency of the AC current:

>>> class GermanToUSSocketAdapterAndTransformer(object):
...     implements(IUSSocket)
...     __used_for__ = IGermanSocket
...
...     def __init__(self, socket):
...         self.context = socket

Now, we need a way to keep the two adapters apart. Thus we register them with a name:

>>> gsm.registerAdapter(GermanToUSSocketAdapter,
...                     (IGermanSocket,), IUSSocket, 'shaver',)
>>> gsm.registerAdapter(GermanToUSSocketAdapterAndTransformer,
...                     (IGermanSocket,), IUSSocket, 'dvd')

Now we simply look up the adapters using their labels (called name):

>>> socket = zope.component.getAdapter(bathroomDE, IUSSocket, 'shaver')
>>> socket.__class__ is GermanToUSSocketAdapter
True

>>> socket = zope.component.getAdapter(bathroomDE, IUSSocket, 'dvd')
>>> socket.__class__ is GermanToUSSocketAdapterAndTransformer
True

Clearly, we do not have an adapter for the MP3 player

>>> zope.component.getAdapter(bathroomDE, IUSSocket, 'mp3') \
... 
Traceback (most recent call last):
...
ComponentLookupError: (<instance of GermanSocket>,
                        <InterfaceClass __builtin__.IUSSocket>,
                        'mp3')

but you could use the ‘dvd’ adapter in this case of course. ;)

Sometimes you want to know all adapters that are available. Let’s say you want to know about all the adapters that convert a German to a US socket type:

>>> sockets = list(zope.component.getAdapters((bathroomDE,), IUSSocket))
>>> len(sockets)
3
>>> names = [name for name, socket in sockets]
>>> names.sort()
>>> names
[u'', u'dvd', u'shaver']

zope.component.getAdapters() returns a list of tuples. The first entry of the tuple is the name of the adapter and the second is the adapter itself.

Multi-Adapters

After watching all the DVDs you brought at least twice, you get tired of them and you want to listen to some music using your MP3 player. But darn, the MP3 player plug has a ground pin and all the adapters you have do not support that:

>>> class IUSGroundedSocket(IUSSocket):
...     pass

So you go out another time to buy an adapter. This time, however, you do not buy yet another adapter, but a piece that provides the grounding plug:

>>> class IGrounder(Interface):
...     pass

>>> class Grounder(object):
...     implements(IGrounder)
...     def __repr__(self):
...         return '<instance of Grounder>'

Then together they will provided a grounded us socket:

>>> class GroundedGermanToUSSocketAdapter(object):
...     implements(IUSGroundedSocket)
...     __used_for__ = (IGermanSocket, IGrounder)
...     def __init__(self, socket, grounder):
...         self.socket, self.grounder = socket, grounder

You now register the combination, so that you know you can create a IUSGroundedSocket:

>>> gsm.registerAdapter(GroundedGermanToUSSocketAdapter,
...                 (IGermanSocket, IGrounder), IUSGroundedSocket, 'mp3')

Given the grounder

>>> grounder = Grounder()

and a German socket

>>> livingroom = GermanSocket()

we can now get a grounded US socket:

>>> socket = zope.component.getMultiAdapter((livingroom, grounder),
...                                         IUSGroundedSocket, 'mp3')
>>> socket.__class__ is GroundedGermanToUSSocketAdapter
True
>>> socket.socket is livingroom
True
>>> socket.grounder is grounder
True

Of course, you do not have a ‘dvd’ grounded US socket available:

>>> zope.component.getMultiAdapter((livingroom, grounder),
...                                IUSGroundedSocket, 'dvd') \
... 
Traceback (most recent call last):
...
ComponentLookupError: ((<instance of GermanSocket>,
                        <instance of Grounder>),
                        <InterfaceClass __builtin__.IUSGroundedSocket>,
                        'dvd')
>>> socket = zope.component.queryMultiAdapter(
...     (livingroom, grounder), IUSGroundedSocket, 'dvd', marker)
>>> socket is marker
True

Again, you might want to read adapter.txt in zope.interface for a more comprehensive coverage of multi-adapters.

Subscribers

While subscribers are directly supported by the adapter registry and are adapters for all theoretical purposes, practically it might be better to think of them as separate components. Subscribers are particularly useful for events.

Let’s say one of our adapters overheated and caused a small fire:

>>> class IFire(Interface):
...     pass

>>> class Fire(object):
...     implements(IFire)

>>> fire = Fire()

We want to use all available objects to put out the fire:

>>> class IFireExtinguisher(Interface):
...     def extinguish():
...         pass

>>> class FireExtinguisher(object):
...     def __init__(self, fire):
...         pass
...     def extinguish(self):
...         "Place extinguish code here."
...         print 'Used ' + self.__class__.__name__ + '.'

Here some specific methods to put out the fire:

>>> class PowderExtinguisher(FireExtinguisher):
...     pass
>>> gsm.registerSubscriptionAdapter(PowderExtinguisher,
...                                 (IFire,), IFireExtinguisher)

>>> class Blanket(FireExtinguisher):
...     pass
>>> gsm.registerSubscriptionAdapter(Blanket, (IFire,), IFireExtinguisher)

>>> class SprinklerSystem(FireExtinguisher):
...     pass
>>> gsm.registerSubscriptionAdapter(SprinklerSystem,
...                                 (IFire,), IFireExtinguisher)

Now let use all these things to put out the fire:

>>> extinguishers = zope.component.subscribers((fire,), IFireExtinguisher)
>>> extinguishers.sort()
>>> for extinguisher in extinguishers:
...     extinguisher.extinguish()
Used Blanket.
Used PowderExtinguisher.
Used SprinklerSystem.

If no subscribers are found for a particular object, then an empty list is returned:

>>> zope.component.subscribers((object(),), IFireExtinguisher)
[]

Utilities

Utilities are the second type of component, the component architecture implements. Utilities are simply components that provide an interface. When you register an utility, you always register an instance (in contrast to a factory for adapters) since the initialization and setup process of a utility might be complex and is not well defined. In some ways a utility is much more fundamental than an adapter, because an adapter cannot be used without another component, but a utility is always self-contained. I like to think of utilities as the foundation of your application and adapters as components extending beyond this foundation.

Back to our story...

After your vacation is over you fly back home to Tampa, Florida. But it is August now, the middle of the Hurricane season. And, believe it or not, you are worried that you will not be able to shave when the power goes out for several days. (You just hate wet shavers.)

So you decide to go to your favorite hardware store and by a Diesel-powered electric generator. The generator provides of course a US-style socket:

>>> class Generator(object):
...     implements(IUSSocket)
...     def __repr__(self):
...         return '<instance of Generator>'

>>> generator = Generator()

Like for adapters, we now have to add the newly-acquired generator to our inventory by registering it as a utility:

>>> gsm.registerUtility(generator, IUSSocket)

We can now get the utility using

>>> utility = zope.component.getUtility(IUSSocket)
>>> utility is generator
True

As you can see, it is very simple to register and retrieve utilities. If a utility does not exist for a particular interface, such as the German socket, then the lookup fails

>>> zope.component.getUtility(IGermanSocket)
Traceback (most recent call last):
...
ComponentLookupError: (<InterfaceClass __builtin__.IGermanSocket>, '')

or more gracefully when specifying a default value:

>>> default = object()
>>> utility = zope.component.queryUtility(IGermanSocket, default=default)
>>> utility is default
True

Note: The only difference between getUtility() and queryUtility() is the fact that you can specify a default value for the latter function, so that it will never cause a ComponentLookupError.

Named Utilities

It is often desirable to have several utilities providing the same interface per site. This way you can implement any sort of registry using utilities. For this reason, utilities – like adapters – can be named.

In the context of our story, we might want to do the following: You really do not trust gas stations either. What if the roads are blocked after a hurricane and the gas stations run out of oil. So you look for another renewable power source. Then you think about solar panels! After a storm there is usually very nice weather, so why not? Via the Web you order a set of 110V/120W solar panels that provide a regular US-style socket as output:

>>> class SolarPanel(object):
...     implements(IUSSocket)
...     def __repr__(self):
...         return '<instance of Solar Panel>'

>>> panel = SolarPanel()

Once it arrives, we add it to our inventory:

>>> gsm.registerUtility(panel, IUSSocket, 'Solar Panel')

You can now access the solar panel using

>>> utility = zope.component.getUtility(IUSSocket, 'Solar Panel')
>>> utility is panel
True

Of course, if a utility is not available, then the lookup will simply fail

>>> zope.component.getUtility(IUSSocket, 'Wind Mill')
Traceback (most recent call last):
...
ComponentLookupError: (<InterfaceClass __builtin__.IUSSocket>, 'Wind Mill')

or more gracefully when specifying a default value:

>>> default = object()
>>> utility = zope.component.queryUtility(IUSSocket, 'Wind Mill',
...                                       default=default)
>>> utility is default
True

Now you want to look at all the utilities you have for a particular kind. The following API function will return a list of name/utility pairs:

>>> utils = list(zope.component.getUtilitiesFor(IUSSocket))
>>> utils.sort()
>>> utils 
[(u'', <instance of Generator>),
   (u'Solar Panel', <instance of Solar Panel>)]

Another method of looking up all utilities is by using getAllUtilitiesRegisteredFor(iface). This function will return an iterable of utilities (without names); however, it will also return overridden utilities. If you are not using multiple site managers, you will not actually need this method.

>>> utils = list(zope.component.getAllUtilitiesRegisteredFor(IUSSocket))
>>> utils.sort()
>>> utils
[<instance of Generator>, <instance of Solar Panel>]

Factories

A factory is a special kind of utility that exists to create other components. A factory is always identified by a name. It also provides a title and description and is able to tell the developer what interfaces the created object will provide. The advantage of using a factory to create an object instead of directly instantiating a class or executing any other callable is that we can refer to the factory by name. As long as the name stays fixed, the implementation of the callable can be renamed or moved without a breakage in code.

Let’s say that our solar panel comes in parts and they have to be assembled. This assembly would be done by a factory, so let’s create one for the solar panel. To do this, we can use a standard implementation of the IFactory interface:

>>> from zope.component.factory import Factory
>>> factory = Factory(SolarPanel,
...                   'Solar Panel',
...                   'This factory creates a solar panel.')

Optionally, I could have also specified the interfaces that the created object will provide, but the factory class is smart enough to determine the implemented interface from the class. We now register the factory:

>>> from zope.component.interfaces import IFactory
>>> gsm.registerUtility(factory, IFactory, 'SolarPanel')

We can now get a list of interfaces the produced object will provide:

>>> ifaces = zope.component.getFactoryInterfaces('SolarPanel')
>>> IUSSocket in ifaces
True

By the way, this is equivalent to

>>> ifaces2 = factory.getInterfaces()
>>> ifaces is ifaces2
True

Of course you can also just create an object:

>>> panel = zope.component.createObject('SolarPanel')
>>> panel.__class__ is SolarPanel
True

Note: Ignore the first argument (None) for now; it is the context of the utility lookup, which is usually an optional argument, but cannot be in this case, since all other arguments beside the name are passed in as arguments to the specified callable.

Once you register several factories

>>> gsm.registerUtility(Factory(Generator), IFactory, 'Generator')

you can also determine, which available factories will create objects providing a certain interface:

>>> factories = zope.component.getFactoriesFor(IUSSocket)
>>> factories = [(name, factory.__class__) for name, factory in factories]
>>> factories.sort()
>>> factories 
[(u'Generator', <class 'zope.component.factory.Factory'>),
   (u'SolarPanel', <class 'zope.component.factory.Factory'>)]

Site Managers

Why do we need site managers? Why is the component architecture API not sufficient? Some applications, including Zope 3, have a concept of locations. It is often desirable to have different configurations for these location; this can be done by overwriting existing or adding new component registrations. Site managers in locations below the root location, should be able to delegate requests to their parent locations. The root site manager is commonly known as global site manager, since it is always available. You can always get the global site manager using the API:

>>> gsm = zope.component.getGlobalSiteManager()

>>> from zope.component import globalSiteManager
>>> gsm is globalSiteManager
True
>>> from zope.component.interfaces import IComponentLookup
>>> IComponentLookup.providedBy(gsm)
True
>>> from zope.component.interfaces import IComponents
>>> IComponents.providedBy(gsm)
True

You can also lookup at site manager in a given context. The only requirement is that the context can be adapted to a site manager. So let’s create a special site manager:

>>> from zope.component.globalregistry import BaseGlobalComponents
>>> sm = BaseGlobalComponents()

Now we create a context that adapts to the site manager via the __conform__ method as specified in PEP 246.

>>> class Context(object):
...     def __init__(self, sm):
...         self.sm = sm
...     def __conform__(self, interface):
...         if interface.isOrExtends(IComponentLookup):
...             return self.sm

We now instantiate the Context with our special site manager:

>>> context = Context(sm)
>>> context.sm is sm
True

We can now ask for the site manager of this context:

>>> lsm = zope.component.getSiteManager(context)
>>> lsm is sm
True

The site manager instance lsm is formally known as a local site manager of context.