Custom content classes

Adding a custom implementation

When we learned about configuring the Dexterity FTI, we saw the klass attribute and how it could be used to refer to either the Container or Item content classes. These classes are defined in the plone.dexterity.content module, and represent container (folder) and item (non-folder) types, respectively.

For most applications, these two classes will suffice. We will normally use behaviors, adapters, event handlers and schema interfaces to build additional functionality for our types. In some cases, however, it is useful or necessary to override the class, typically to override some method or property provided by the base class that cannot be implemented with an adapter override. A custom class may also be able to provide marginally better performance by side-stepping some of the schema-dependent dynamic behavior found in the base classes. In real life, you are very unlikely to notice, though.

Creating a custom class is simple: simply derive from one of the standard ones, e.g.:

from plone.dexterity.content import Item

class MyItem(Item):
    """A custom content class"""

For a container type, we’d do:

from plone.dexterity.content import Container

class MyContainer(Container):
    """A custom content class"""

You can now add any required attributes or methods to this class.

To make use of this class, set the klass attribute in the FTI to its dotted name, e.g.

<property name="klass">my.package.myitem.MyItem</property>

This will cause the standard Dexterity factory to instantiate this class when the user submits the add form.


As an alternative to setting klass in the FTI, you may provide your own IFactory utility for this type in lieu of Dexterity’s default factory (see plone.dexterity.factory). However, you need to be careful that this factory performs all necessary initialisation, so it is normally better to use the standard factory.

Custom class caveats

There are a few important caveats when working with custom content classes:

  • Make sure you use the correct base class: either plone.dexterity.content.Item or plone.dexterity.content.Container.
  • If you mix in other base classes, it is safer to put the Item or Container class first. If another class comes first, it may override the __name__, __providedBy__, __allow_access_to_unprotected_subobjects__ and/or isPrincipiaFolderish properties, and possibly the __getattr__() and __getitem__() methods, causing problems with the dynamic schemata and/or folder item security. In all cases, you may need to explicitly set these attributes to the ones from the correct base class.
  • If you define a custom constructor, make sure it can be called with no arguments, and with an optional id argument giving the name.