Grok-like configuration for basic components (adapters, utilities, subscribers)

pip install grokcore.component==4.1


This package provides base classes of basic component types for the Zope Component Architecture, as well as means for configuring and registering them directly in Python (without ZCML).

How to set up grokcore.component

In the following we assume you're writing or extending an application that does bootstrap configuration using ZCML. There's always a single ZCML file that is executed when the application is started, which then includes everything else. Let's assume this file is called site.zcml (that's what it's called in Zope), so that file is what we'll be editing.

In order to register the components that you wrote using the base classes and directives available from grokcore.component, we'll use the <grok:grok /> ZCML directive. But before we can use it, we need to make sure it's available to the ZCML machinery. We do this by including the meta configuration from grokcore.component:

<include package="grokcore.component" file="meta.zcml" />

Put this line somewhere to the top of site.zcml, next to other meta configuration includes. Now, further down the line, we can tell the machinery in grokcore.component to register all components in your package (let's say it's called helloworld):

<grok:grok package="helloworld" />

To sum up, your site.zcml file should look like something like this:


  <!-- do the meta configuration to make the ZCML directives available -->
  <include package="zope.foobar" file="meta.zcml" />
  <include package="zope.frobnaz" file="meta.zcml" />
  <include package="grokcore.component" file="meta.zcml" />

  <!-- now load the configuration of packages that we depend on -->
  <include package="zope.barfoo" />
  <include package="zope.somethingorother" />

  <!-- finally load my components which are based on grokcore.component -->
  <grok:grok package="helloworld" />


There is an optional exclude on the grok directive. It allows to specify names of packages or modules that if encountered won't be grokked. These names might contain unix shell-style wildcards.

implementer() versus implements()

Note how the Python 3 compatibility brings a change in how the grokcore.component.implements() directive works. When using this directive you now have to make sure the component is grokkked, to have the underlying mechanism to take effect.

Alternatively you could start to use the grokcore.component.implementer() class decorator instead. This will do the same thing, but does not require your component to be grokkked and still have your component declare it implements the given interface.



Here's a simple adapter that may be useful in Zope. It extracts the languages that a user prefers from the request:

import grokcore.component
from zope.publisher.interfaces.browser import IBrowserRequest
from zope.i18n.interfaces import IUserPreferredLanguages

class CookieLanguage(grokcore.component.Adapter):
    """Extract the preferred language from a cookie"""

    # No need to implement __init__, it's already provided by the base class.

    def getPreferredLanguages(self):
        # This an adapter for the request, so self.context is the request.
        request = self.context

        # Extract the preferred language from a cookie:
        lang = request.cookies.get('language', 'en')

        # According to IUserPreferredLanguages, we must return a list.
        return [lang]


Here's a multi-adapter that functions as a content provider as known from the zope.contentprovider library. Content providers are components that return snippets of HTML. They're multi-adapters for the content object (model), the request and the view that they're supposed to be a part of:

import grokcore.component
from zope.publisher.interfaces.browser import IBrowserRequest
from zope.publisher.interfaces.browser import IBrowserPage
from zope.contentprovider.interfaces import IContentProvider

class HelloWorldProvider(grokcore.component.MultiAdapter):
    """Display Hello World!"""
    grokcore.component.adapts(Interface, IBrowserRequest, IBrowserPage)

    def __init__(self, context, request, view):

    def update(self):

    def render(self):
        return u'<p>Hello World!</p>'

Global utility

Here's a simple named utility, again from the Zope world. It's a translation domain. In other words, it contains translations of user messages and is invoked when the i18n machinery needs to translate something:

import grokcore.component
from zope.i18n.interfaces import ITranslationDomain

class HelloWorldTranslationDomain(grokcore.component.GlobalUtility):

    domain = u'helloworld'

    def translate(self, msgid, mapping=None, context=None,
                  target_language=None, default=None):
        if target_language is None:
            preferred = IUserPreferredLanguages(context)
            target_language = preferred.getPreferredLanguages()[0]

        translations = {'de': u'Hallo Welt',
                        'nl': u'Hallo Wereld'}
        return translations.get(target_language, u'Hello World')

Of course, it's silly to implement your own translation domain utility if there are already implementations available in zope.i18n (one that reads translations from a GNU gettext message catalog and a simple implementation for tests). Let's try to reuse that implementation and register an instance:

import grokcore.component
from zope.i18n.interfaces import ITranslationDomain
from zope.i18n.simpletranslationdomain import SimpleTranslationDomain

messages = {('de', u'Hello World'): u'Hallo Welt',
            ('nl', u'Hello World'): u'Hallo Wereld'}
helloworld_domain = SimpleTranslationDomain(u'helloworld', messages)


Global adapter

Sometimes, you may have an object that should be registered as an adapter factory. It may have come from some other framework that configured that adapter for you, say, or you may have a class that you instantiate many times to get different variations on a particular adapter factory. In these cases, subclassing grokcore.component.Adapter or MultiAdapter is not possible. Instead, you can use the global_adapter() directive. Here is an example drawing on the z3c.form library, which provides an adapter factory factory for named widget attributes:

import zope.interface
import zope.schema
import grokcore.component
import z3c.form.widget import ComputedWidgetAttribute

class ISchema(Interface):
    """This schema will be used to power a z3c.form form"""

    field = zope.schema.TextLine(title=u"Sample field")


label_override = z3c.form.widget.StaticWidgetAttribute(
                      u"Override label", field=ISchema['field'])

grokcore.component.global_adapter(label_override, name=u"label")

In the example above, the provided and adapted interfaces are deduced from the object returned by the StaticWidgetAttribute factory. The full syntax for global_adapter is:

global_adapter(factory, (IAdapted1, IAdapted2,), IProvided, name=u"name")

The factory must be a callable (the adapter factory). Adapted interfaces are given as a tuple. You may use a single interface instead of a one-element tuple for single adapters. The provided interface is given as shown. The name defaults to u"" (an unnamed adapter).

Handling events

Here we see an event handler much like it occurs within Zope itself. It subscribes to the modified event for all annotatable objects (in other words, objects that can have metadata associated with them). When invoked, it updates the Dublin Core 'Modified' property accordingly:

import datetime
import grokcore.component
from zope.annotation.interfaces import IAnnotatable
from zope.lifecycleevent.interfaces import IObjectModifiedEvent
from zope.dublincore.interfaces import IZopeDublinCore

@grokcore.component.subscribe(IAnnotatable, IObjectModifiedEvent)
def updateDublinCoreAfterModification(obj, event):
    """Updated the Dublin Core 'Modified' property when a modified
    event is sent for an object."""
    IZopeDublinCore(obj).modified = datetime.datetime.utcnow()


Subscriptions look similar to Adapter, however, unlike regular adapters, subscription adapters are used when we want all of the adapters that adapt an object to a particular adapter.

Analogous to MultiAdapter, there is a MultiSubscription component that "adapts" multiple objects.