Grok Developer's Notes

This document is a developer's overview of Grok. It is not intended to be a beginner's tutorial. It's also not a reference. It gives a succinct an overview of what's there in Grok, with a brief idea on how to use it, so you can try it out and learn more about it. This includes rules, common APIs and patterns.


A Grok-based application is composed out of one or more models. We also call these content objects, or just objects. The objects are just Python objects, instantiated from a class. Models can be stored in the object database (ZODB), created by an object relational mapper, or created on the fly by your code.

Grok comes with two kinds of models: grok.Model and grok.Container. grok.Model is the most basic one and doesn't really do much for you. You can subclass from grok.Model, like this:

class Document(grok.Model):

The main thing subclassing from grok.Model does is make it possible (but not required) to store instances in the ZODB.

You can also subclass from grok.Container, like this:

class Folder(grok.Container):

A container is like a model, but also acts much like a Python dictionary. The main difference with Python dictionaries is that its methods, like keys and items, are iterator-like. They also do more, like send events, but we can forget about that for now.

In order to be able to install an application, you need to mix in grok.Application into a class:

class Application(grok.Application, grok.Container):

Instances of this class can now be installable in the Grok web UI.

Let's make a structure with some folders and documents:

app = Application()
app['a'] = a = Container()
a['b'] = Document()
a['c'] = Container()
a['c']['d'] = Document()

Grok publishes these objects to the web: this is called object publishing. What this means in essence is that objects can be addressed with URLs. When you access a URL of a Grok application with your web browser, Grok uses this URL to find an object.

An example: if app were installed under http://localhost:8080/app, the following URLs will exist in your application:


__parent__ and __name__

Models in Grok are automatically supplied with a __parent__ and a __name__ attribute.

  • __parent__ points to object this object is in. If the object is in a container, this is the container.
  • __name__ is the name this object has in a URL (and in its container, if it is in a container).

These attributes are used for navigation through content space, and Grok also uses them to construct URLs automatically (see below). The __parent__ and __name__ attributes are automatically added to an object when it is placed in a container, or when it is being traversed through using traverse.

Custom traversing

Grok resolves URLs to objects by traversing through the containers and models in question. What if you want to customize the way this traversal works? Perhaps you want to traverse through objects you create yourself, or objects created by an object relational mapper. Grok offers a handy way to do so: the traverse method on models.

A math example: imagine you want to create an application that represents integer numbers, and you want to traverse to each individual number, like this:


and so on. How would we implement this? We cannot create a container and fill it with all integers possible, as there are an infinite number of them. Okay, so we are in a math example, so let's be exact: this is true if we ignore memory limitations and URL length limitations. Storing all possible integers in a container is just not practical.

We use the traverse method:

class ParticularInteger(grok.Model):
    def __init__(self, number):
        self.number = number

class Integers(grok.Application, grok.Model):
    def traverse(self, name):
             value = int(name)
        except ValueError:
             return None # not an integer
        return ParticularInteger(value)

Now all URLs for numbers are available. What's more, other URLs like this are not:


The traverse method works by trying to convert the path element that comes in as name to an integer. If it fails, we return None, telling Grok that the traverse() method didn't find the object asked for. Grok then falls back on default behavior, which in this case would mean a 404 Not Found error.


Now that we have models and can build structures of them, we will need to look at ways to actually present them to the user: views. So what is a view? A view is a class that represents a model in some way. It creates a user interface of some sort (typically HTML) for a model. A single model can have more than one view. It looks like this:

class Index(grok.View):

    def render(self):
        return "This is the application"

The grok.context bit in the class is an example of using a Grok directive. If you use grok.context on a view class, it connects the view to the class we give it. So in this case, Index is a view for Application. Note that if there is only a single model in the module and you want your view to be associated with it, you can leave out grok.context and the view will be associated with that model by default. Many directives have such default behavior, allowing you to leave them out of your code if you organize your code in a certain way.

The default view for a model is called index. You can specify index at the end of the URL, like this:


What happens when you go to this URL is that Grok instantiates the Index class, creating a Index instance. View instances have a number of attributes by default:

  • context, the model instance that the view is presenting.

  • request, the current web request.

  • response, an object representing the response sent to the

    user. Used less often.

index views are special, as it's also fine not to add index at the end, because the name index is the default:


You can also create views with different names:

class Edit(grok.View):

    def render(self):
        return "This is the edit screen for the application"

Now you can go to this URL:


The name of the view is the name of the view class, lowercased. This is the default behavior: you can override this using the directive:

class SomeImpossiblyLongClassName(grok.View):

    def render(self):
        return "This is the edit screen for the application"


In the previous examples, we used the render method to determine what you actually see on a web page. For most views we don't want to do that: we want to use a template to prepare presentation. Using a template with a view is easy. First create a directory <name>_templates, where <name> is the the module that contains the views. So, if you are developing in a module, you need to create a subdirectory app_templates for templates in the same directory as the module.

You can then add templates to that directory with the same name as the view class name (lowercase), with the .pt extension appended. These templates follow the Zope Page Template (ZPT) rules, though Grok can also be extended to support other template languages.

You could for instance have this view:

class Index(grok.View):

and a file in the module's templates directory containing template code.

These are the defaults. If for some reason you want the name of the template directory not to be based on the name of module, you can manually set the name of the template directory used by a module by using the grok.templatedir directive in the module. If you want the name of the template not to be based on the name of the class, you use the grok.template directive in the view class.

The template can access attributes and methods on the view through the special view name available in the template. The template can access attributes and methods on the model through the special context name available in the template. The template has the following special names available:

* ``view`` - the view that this template is associated with
  • context - the model that is being viewed
  • request - the current request object
  • static - to make URLs to static content made available by this module

and any names you also make available using the namespace method.

static content

A typical web page references one or more CSS files, javascript files and images: static content that is part of the layout.

To make available static content to your template create a directory in your package called static. Put .css files, .js files, image and whatever else is needed in there.

You can now refer to these static files in your template using the special name static, like this (ZPT example):

<img tal:attributes="src static/my_image.png" />

This will automatically create a URL to the place where Grok published that image.

You can create subdirectories in static and refer to them as you'd expect:

<image tal:attributes="src static/images/some_image.gif" />


You can define an update method in a view to prepare a view just before it is accessed. You can use this to process information in the request (URL parameters or form variables) or in the context, and set attributes on the view that can be used in the template:

def update(self): = int(self.request.form['a']) + int(self.request.form['b'])

The template now has access to

You can define parameters in the update view. These will be automatically bound to parameters (or form values) in the request:

def update(self, a, b): = int(a) + int(b)


If you just want a variable to become available in the top-level of your template (much like view and model), you can also define the namespace method on the view:

def namespace(self):
    return {'foo': "Some value"}

You can now refer to foo in your template and have available to this value.

the url method

Views have a special method called url() that can be used to create URLs to objects. The url method takes zero, one or two arguments:

* self.url() - URL to this view.
  • self.url(object) - URL to the provided object.

  • self.url(u"name") - URL to the context object, with /name appended,

    to point to a view or subobject of the context.

  • self.url(object, u"name") - URL to the provided object, with

    /name appended, to point to a view or subobject of the provided object.

From the view, this is accessed through self.url(). From the template, this method can be accessed using view.url().

the application_url method

When using views it is sometimes desirable to be able to construct a URL to the application object. application_url is a quick way to do it. It takes a single optional argument, name, which is the name of a view of the application.

the redirect method

The redirect method on views can be used to redirect the browser to another URL. Example:

def render(self):
    # return empty body as we are going to redirect anyway
    return ''

__parent__ and __name__ on views

Like models, views also get supplied with a __parent__ and __name__ object when they are instantiated for a particular model.

__parent__ points to the model being viewed (and is the same as context, which should normally be used).

__name__ is the name of the view in the URL.

The @@ thing

Supposing you have a view called edit, whenever you write this:


you can also write this:


Why the ugly @@ syntax? Imagine that app is a container, and that your user interface lets the user add objects to it with a name of their own choosing. The user could decide to add an object called index. In that case Grok wouldn't know whether the http://localhost:8080/app/index index is to get to a view or a subobject. @@ tells the system to look up a view definitely. If @@ is not provided, subobjects take precedence over views in case of name collision.


Some useful things to know about the request object (accessible as an attribute on the view):

Information on the request object can be accessed using mapping access (request`foo`). You can access request form variables and cookies and headers (including environment variables).

To access form variables in particular use: request.form'foo'.

To access cookies in particular use: request.cookies'foo'.

To access headers (and environment variables) in particular use: request.headers'foo'. You can also use request.getHeader(), with the header name as the argument, and an optional second default argument.

Instead of the mapping access, the get methods work as well, as on normal Python dictionaries.

More can be found in the IHTTPRequest interface documentation in zope.publisher.interfaces.http.


Some useful things to know about the response object (accessible as an attribute on the view):

setStatus(name, reason) sets the HTTP status code. The argument may either be an integer representing the status code (such as 200 or 400), or a string (OK, NotFound). The optional second argument can be be used to pass the human-readable representation (Not Found).

setHeader(name, value) can be used to set HTTP response headers. The first argument is the header name, the second the value.

addHeader(name, value) can be used to add a HTTP header, while retaining any previously set headers with the same name.

setCookie(name, value, **kw) can be used to set a cookie. The first argument is the cookie name, the second the value. Optional keyword arguments can be used to set up further cookie properties (such as max_age and expires).

expireCookie(name, value) can be used to immediately expire a cookie.

More can be found in the IHTTPResponse interface documentation in zope.publisher.interfaces.http.


An adapter is much like a view, but is aimed towards developers, not end users. It presents an interface to an object, but an interface for developers, not an user interface for end-users.

The section on adapters will of necessity be rather abstract. Feel free to skip it until you want to know what is going on up with interfaces and adapters - it is an important foundation to Grok, but one you do not know much about when you get started.

An adapter can be used to add new methods to an object without changing the object. To demonstrate the principle, we will construct adapters entirely by hand first. At the end we will show how Groks helps in constructing adapters and using them.

Imagine we are developing a content management system and we want to get information about the size (in, say, bytes, approximately) of content objects stored in our CMS, for instance in order to display it in our UI or to calculate the total size of all objects in a container. The simplest approach would be to add a size() method to all our content objects:

class Document(grok.Model):
     def __init__(self, text):
         self.text = text

     def size(self):
         return len(self.text.encode('UTF-8'))

class Image(grok.Model):
     def __init__(self, data):
 = data

     def size(self):
          return len(

class Container(grok.Container):
      def size(self):
          total = 0
          for obj in self.values():
              total += obj.size()
          return total

For simple cases this is fine, but for larger applications this can become a problem. Our Document model needs a size method, and does our Image model, and our Container, and our News Item model, and so on. Given the requirements of a typical CMS, content objects would soon end up with a very large number of methods, for all sorts of functionality, from getting the size of objects to offering a commenting facility. It would be nicer to separate things out and keep the underlying models clean.

To do this, we can use the adaptation pattern. As said, we will do it by hand at first. An adapter is an object that adds an API to another object (typically stored as the context attribute of the adapter):

class DocumentSized(object):
    def __init__(self, context):
        self.context = context

    def size(self):
        return len(self.context.text.encode('UTF-8'))

We would use it like this:


We could extend this same adapter to work for different kinds of content objects, but that isn't very extensible when new adapters need to be made:

class Sized(object):
    def __init__(self, context):
        self.context = context

    def size(self):
        if isinstance(self.context, Document):
             return len(self.context.text.encode('UTF-8'))
        elif isinstance(self.context, Image):
             return len(
        elif isintance(self.context, Container):
             total = 0
             for obj in self.context.values():
                 total += Sized(obj).size()
             return total

Instead, we can create a smart sized factory that does this switch-on-type behavior instead, keeping our adapters clean:

class DocumentSized(object):
    def __init__(self, context):
        self.context = context

    def sized(self):
        return len(self.context.text.encode('UTF-8'))

class ImageSized(object):
    def __init__(self, context):
        self.context = context

    def sized(self):
        return len(

class ContainerSized(object):
    def __init__(self, context):
        self.context = context

    def sized(self):
        total = 0
        for obj in self.context.values():
            total += sized(obj).size()
        return total

def sized(context):
    if isinstance(context, Document):
        return DocumentedSized(context)
    elif isinstance(context, Image):
        return ImageSized(context)
    elif isinstance(context, Container):
        return ContainerSized(context)

We can now call sized for a content object and get an object back that implements the "sized API":

s = sized(my_content_object)
print s.size()

It's good to spell out the APIs of your application explicitly, as documentation so that other developers can work with them and also implement them for their own content objects. Grok lets you do this using an interface specification, using the zope.interface package:

from zope.interface import Interface

class ISized(Interface):
    def size():
         "Return the size of the object"

We can now make this ISized interface into the adapter factory (like sized above), without actually having to implement it directly. Let's do that now by subclassing from grok.Adapter and using a few grok directives:

class DocumentSized(grok.Adapter):

    def sized(self):
        return len(self.context.text.encode('UTF-8'))

class ImageSized(grok.Adapter):

    def sized(self):
        return len(

class ContainerSized(grok.Adapter):

    def sized(self):
        total = 0
        for obj in self.context.values():
            total += ISized(obj).size()
        return total

We can now use ISized like we used sized above:

s = ISized(my_content_object)
print s.size()

When new content objects were to be created for this CMS, ISized adapters can be registered for them anywhere. Using this pattern, existing objects implemented by someone else can be made to conform with the ISized API without having to modify them.

grok.context works as for views. It is useful to point it to any class however, not just that of models. grok.provides has to be pointed to an interface (the interface that the adapter adapts to).


Classes can also be made to implement an interface. This means that instances of that class provide that interface:

from zope.interface import Interface, Attribute

class IAnimal(Interface):
    name = Attribute("The name of the animal")

    def makeSound():
        "The sound the animal makes."

class Cow(object):

    def __init__(self, name): = name

    def makeSound(self):
        return "Mooo"

We can ask the interface machinery whether an object provides an interface:

>>> cow = Cow()
>>> IAnimal.providedBy(cow)

If you use an interface to adapt an object, and this object already provides the interface, you get back the object itself:

>>> IAnimal(cow) is cow

grok.context can always point to an interface instead of a class directly. This indirection can be useful to make a view or adapter work for a whole set of classes that all implement the same interface.


What if an adapter cannot be found for a particular object? Perhaps no adapter has been registered for a particular object or a particular interface. The system will raise a ComponentLookupError:

>>> ISized(cow)
Traceback (most recent call last):

If you want to catch this exception, you can import it from zope.component.interfaces:

from zope.component.interfaces import ComponentLookupError

Named adapters

It is possible to give an adapter a name, making it a named adapter. This way it is possible to have more than one adapter registered for a single object that all provide the same interface, each with a different name. This feature is rarely used directly, but internally it is used for views, as we will see later. The directive can be used to give an adapter a name:

class Adapter(object):'somename')

Actually all adapters are named: by default the name of an adapter is the empty string.

You cannot call the interface directly to get a named adapter for an object. Instead, you need to use the APIs provided by the zope.component package, in particular getAdapter:

from zope import component

my_adapter = component.getAdapter(some_object, ISomeInterface,

getAdapter can also be used to look up unnamed adapters, as an alternative to using the interface directly:

myadapter = component.getAdapter(some_object, ISomeInterface)

Multi adapters

Another feature of adapters is that you can adapt multiple objects at once using a multi adapter. Again this feature is rarely used in practice, except internally to implement views and events.

You can construct a multi adapter by subclassing from grok.MultiAdapter:

class MyMultiAdapter(grok.MultiAdapter):
    grok.adapts(SomeClass, AnotherClass)

    def __init__(some_instance, another_instance):
        self.some_interface = some_instance
        self.another_instance = another_instance

The multi-adapter receives as many arguments as what it was registered for using grok.adapts.

A multi adapter also cannot be looked up directly by calling the interface. Instead, we need to use the zope.component package again:

from zope import component

my_multi_adapter = component.getMultiAdapter((some_object, another_object),

getMultiAdapter receives as the first argument a tuple with the combination of objects to adapt.

It can also optionally be named using and then looked up using a name argument:

my_named_multi_adapter = component.getMultiAdapter(
    (some_object, another_object), ISomeInterface, name="foo")

Views as adapters

A view in Grok is in fact a named multi adapter, providing the base interface (Interface). This means that a view in Grok can be looked up in code by the following call:

from zope.interface import Interface

view = component.getMultiAdapter((object, request), Interface, name="index")

Since the default for the second argument is in fact Interface, this call can be shorted to this:

view = component.getMultiAdapter((object, request), name="index")

Being able to do this in code is sometimes useful. It is also what Grok does internally when it looks up a view.


Grok lets you write handlers for events. Using event handlers you can hook into code that you do not control. Events allow decoupling: a framework can send events without worrying who is interested in it, and similarly you can send events to work with existing bits of framework that expects them. You can also define new types of events if you are designing a framework yourself.

You write an event handler by writing a function that subscribes to the event, and marking it with a python decorator:

@grok.subscribe(Document, grok.IObjectAddedEvent)
def handle(obj, event):
    print "Object %s was added." % obj

Whenever an instance of a model of class Document (or subclasses) is added to a container, this code will be run. You can then take some action. Any grok.Container subclass will take care of sending these events automatically. You can have as many subscribers for a particular event as you like. The order in which they are run is not guaranteed by the system, so cannot be relied on.

The event handler takes two arguments: the object for which the event was fired, and the event instance. The event instance has attributes, depending on the type of event.

Events defined by Grok

Here we describe the standard events defined by Grok. Described are the interfaces which you would use in a subscriber, and how you can send this event yourself. Other events may be defined by libraries or by you.


Will be fired whenever an object is moved from container to container, renamed, added or removed.

The event object has these attributes:

  • object - the object being moved

  • oldParent - the parent (container) from which the object was moved

    or removed, or None if this object is newly added.

  • oldName - the previous name of the object in its container,

    before renaming if renaming took place, or None if this object is newly added.

  • newParent - the parent (container) to this object was moved or

    added. None if this object was removed.

  • newName - the name the object has in the new container, or None

    if this object was removed.

Containers take care of sending this event, but should you want to send it yourself, use:

grok.notify(grok.ObjectMovedEvent(obj, oldParent, oldName, newParent, newName))

Fired when an object is added to a container. Specialization of IObjectMovedEvent, and shares the attributes as described.

Containers take care of sending this event, but should you want to send it yourself, use:



grok.notify(grok.ObjectAddedEvent(obj, newParent, newName))

Fired when an object is removed from a container (and not re-added elsewhere). Specialization of IObjectMovedEvent, and shares the attributes as described.

Containers take care of sending this event, but should you want to send it yourself, use:



grok.notify(grok.ObjectRemovedEvent(obj, oldparent, oldName))

Fired when an object is modified by the system, such as when a form is saved. If you modify the object in code, the system won't know about this, and you will have to remember to send it yourself.

This event has a single attribute, object, which is the object that was modified.

To send this event yourself, use:


A specialization of IObjectModifiedEvent that fires when the container was modified by adding something to it or removing from it.

Containers take care of sending this event, but if you want to send it yourself, use:


Fired when an object is created. When you create your own objects the system won't know about this, and you will have to remember to send it yourself if you care about listing to IObjectCreatedEvent. This is fairly rare - usually you're better of looking at IObjectAddedEvent if you can.

This event has a single attribute, object, which is the object that was created.

To send this event yourself:


Fired when an object was copied. It is a specialization of IObjectCreatedEvent that is fired by the system if you use the zope.copypastemove functionality.

Besides the object attribute it shares with IObjectCreattedEvent, it has also has the original attribute, which was the object that iwas copied from.

To send this event yourself:

grok.notify(grok.ObjectCopiedEvent(copy, original))

Creating and sending your own events

If you are going to send an object that pertains to a particular object, subclass zope.component.interfaces.ObjectEvent:

from zope.component.interfaces import ObjectEvent

class MyEvent(ObjectEvent):

You can then send it like this:


And listen for it like this:

grok.subscribe(SomeClass, MyEvent)
def handle_my_event(obj, event):

This subclassing from ObjectEvent is not required; if your event isn't about an object, you can choose to design your event class entirely yourself. See zope.sendmail for the construction of mail sending events for an example.

Interfaces for events

For documentation purposes it can be a good idea to to define an interface for your event. You can then also allow for multiple implementations of the same event interface. When you have an interface for your event, you can then listen for the interface in the subscribers as well:

from zope.interface import Interface

class IMyEvent(zope.component.interfaces.IObjectEvent):
    "My special event"

class MyEvent(zope.component.interfaces.ObjectEvent):

grok.subscribe(SomeClass, IMyEvent):
def handle_my_event(obj, event):

More about interfaces

We have seen small examples of interfaces before, but here we will go a bit more into them, and why they are useful.

An interface is a description of the API of a class (or more rarely, module or object). Interfaces are useful because:

  • They are API documentation.
  • They can describe how a framework expects you to implement classes that fit into it.
  • The system can inspect the interfaces a particular object provides, and treat them as an abstract form of classes for registration purposes.

Interfaces make it possible to use a generic framework's pluggability points with confidence: you can clearly see what you are supposed to implement to plug into it. You can define very generic frameworks yourself by defining them in terms of interfaces.

Some interface features

A summary of interface features we've seen:

  • To create an interface, subclass from zope.interface.Interface.

  • To state that implementors of the interface must have a method, supply the method with arguments. Don't use self as the first arguments, as this is an implementation detail not important to the interface. Instead, describe the methods as they look to the caller.

  • To state that implementors of the interface must have an attribute, use:

    some_attribute = zope.interface.Attribute("Description of attribute")
  • To state a class implements an interface, use grok.implements.

  • Instances of a class are said to provide the interface that the class implements.

  • You can check whether an instance provides a certain interface by using some_interface.providedBy:


Interfaces and events

Let's study interfaces some more in connection with IObjectModifiedEvent. The IObjectModifiedEvent interface looks like this:

class IObjectModifiedEvent(zope.component.interfaces.IObjectEvent):
    """An object has been modified"""

This refers us to the IObjectEvent interface, which looks like this:

from zope import interface

class IObjectEvent(interface.Interface):
    """An event related to an object.

    object = interface.Attribute("The subject of the event.")

We therefore know that if we implement IObjectModifiedEvent, we must supply a single attribute, object.

The following event handler for instances of SomeClass subscribes to any event that provides IModifiedObjectEvent:

@grok.subscribe(SomeClass, IObjectModifiedEvent):
def handle_event(obj, event):
    "Called when there is an IObjectModifiedEvent for SomeClass instances."

This handler will be called not only for subclasses of the grok.ObjectModifiedEvent class, but also for other, otherwise unrelated classes that implement IObjectEvent, such as this one:

class NonSubclassObjectEvent(object):

    def __init__(self, object):
         self.object = object

So far we have only used interfaces for the second argument of the event handler registration, but the principle also works for the first argument. For example, to handle IObjectModifiedEvent events for all kinds of containers, you can subscribe to objects:

@grok.subscribe(IContainer, IObjectModifiedEvent):
def handle_event(obj, event):
    "Called whenever any container is modified" defines the abstract container API that all containers must provide, no matter how they are implemented internally.

Interfaces and adapters

The same principle also works for adapters and grok.context. You can use grok.context with interfaces as well as with concrete classes. To write an adapter that works for any kind of container, you can write:

from import IContainer

class SortedKeysAdapter(grok.Adapter):

    def sortedKeys(self):
        return sorted(self.context.keys())

Interfaces and views

The same principle can also be used with grok.context in other places, such as in views. This view is registered for all containers:

from import IContainer

class Keys(grok.View):

   def render(self):
       return ', '.join(ISortedKeysAdapter(self.context).sortedKeys())

The view keys exists for all containers, no matter how they are implement, where they are implemented or who implemented them, as long as they provide IContainer.

Using the fact Interface is the base of all interfaces, you can even register a view for all objects. This can be useful to register ZPT macros, which will then be available on all contexts:

class Layout(grok.View):

with a template associated to it.

You can then use these macros in any page template anywhere by referring to them like this:

<html metal:use-macro="context/@@layout/macros/page">