Containment constraints

Containment constraints allow us to express restrictions on the types of items that can be placed in containers or on the types of containers an item can be placed in. We express these constraints in interfaces. We will use some container and item interfaces defined in zope.container.tests.constraints_example:

from zope.interface import implementer
from zope.location.interfaces import IContained

from zope.container.constraints import containers
from zope.container.constraints import contains
from zope.container.interfaces import IContainer

class IBuddyFolder(IContainer):

In this example, we used the contains function to declare that objects that provide IBuddyFolder can only contain items that provide IBuddy. Note that we used a string containing a dotted name for the IBuddy interface. This is because IBuddy hasn’t been defined yet. When we define IBuddy, we can use IBuddyFolder directly:

Now, with these interfaces in place, we can define Buddy and BuddyFolder classes:


class Buddy:

and verify that we can put buddies in buddy folders:

>>> from zope.component.factory import Factory
>>> from zope.container.constraints import checkFactory
>>> from zope.container.constraints import checkObject
>>> from zope.container.tests.constraints_example import Buddy
>>> from zope.container.tests.constraints_example import BuddyFolder

>>> checkObject(BuddyFolder(), 'x', Buddy())
>>> checkFactory(BuddyFolder(), 'x', Factory(Buddy))

If we try to use other containers or folders, we’ll get errors:

>>> from zope.container.interfaces import IContainer
>>> from zope.interface import implementer
>>> @implementer(IContainer)
... class Container:
...     pass

>>> from zope.location.interfaces import IContained
>>> @implementer(IContained)
... class Contained:
...     pass

>>> checkObject(Container(), 'x', Buddy())
Traceback (most recent call last):
zope.container.interfaces.InvalidContainerType: ...

>>> checkFactory(Container(), 'x', Factory(Buddy))

>>> checkObject(BuddyFolder(), 'x', Contained())
Traceback (most recent call last):
zope.container.interfaces.InvalidItemType: ...

>>> checkFactory(BuddyFolder(), 'x', Factory(Contained))

In the example, we defined the container first and then the items. We could have defined these in the opposite order:

class BuddyFolder:

class IContact(IContained):

class IContacts(IContainer):

>>> from zope.container.tests.constraints_example import Contact
>>> from zope.container.tests.constraints_example import Contacts
>>> checkObject(Contacts(), 'x', Contact())

>>> checkFactory(Contacts(), 'x', Factory(Contact))

>>> checkObject(Contacts(), 'x', Buddy())
Traceback (most recent call last):
zope.container.interfaces.InvalidItemType: ...

>>> checkFactory(Contacts(), 'x', Factory(Buddy))

The constraints prevent us from moving a container beneath itself (either into itself or another folder beneath it):

>>> container = Container()
>>> checkObject(container, 'x', container)
Traceback (most recent call last):
TypeError: Cannot add an object to itself or its children.

>>> from zope.interface import directlyProvides
>>> from zope.location.interfaces import ILocation
>>> subcontainer = Container()
>>> directlyProvides(subcontainer, ILocation)
>>> subcontainer.__parent__ = container
>>> checkObject(subcontainer, 'x', container)
Traceback (most recent call last):
TypeError: Cannot add an object to itself or its children.