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=========================
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Containment constraints
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=========================
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Containment constraints allow us to express restrictions on the types
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of items that can be placed in containers or on the types of
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containers an item can be placed in. We express these constraints in
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interfaces. Let's define some container and item interfaces:
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>>> from zope.container.interfaces import IContainer
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>>> from zope.location.interfaces import IContained
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>>> from zope.container.constraints import containers, contains
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>>> class IBuddyFolder(IContainer):
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... contains('.IBuddy')
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In this example, we used the contains function to declare that objects
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that provide IBuddyFolder can only contain items that provide IBuddy.
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Note that we used a string containing a dotted name for the IBuddy
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interface. This is because IBuddy hasn't been defined yet. When we
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define IBuddy, we can use IBuddyFolder directly:
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>>> class IBuddy(IContained):
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... containers(IBuddyFolder)
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Now, with these interfaces in place, we can define Buddy and
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BuddyFolder classes and verify that we can put buddies in buddy
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>>> from zope import interface
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>>> @interface.implementer(IBuddy)
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>>> @interface.implementer(IBuddyFolder)
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... class BuddyFolder:
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>>> from zope.container.constraints import checkObject, checkFactory
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>>> from zope.component.factory import Factory
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>>> checkObject(BuddyFolder(), 'x', Buddy())
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>>> checkFactory(BuddyFolder(), 'x', Factory(Buddy))
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If we try to use other containers or folders, we'll get errors:
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>>> @interface.implementer(IContainer)
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>>> @interface.implementer(IContained)
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>>> checkObject(Container(), 'x', Buddy())
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... # doctest: +ELLIPSIS
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Traceback (most recent call last):
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InvalidContainerType: ...
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>>> checkFactory(Container(), 'x', Factory(Buddy))
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>>> checkObject(BuddyFolder(), 'x', Contained())
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... # doctest: +ELLIPSIS
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Traceback (most recent call last):
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>>> checkFactory(BuddyFolder(), 'x', Factory(Contained))
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In the example, we defined the container first and then the items. We
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could have defined these in the opposite order:
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>>> class IContact(IContained):
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... containers('.IContacts')
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>>> class IContacts(IContainer):
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... contains(IContact)
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>>> @interface.implementer(IContact)
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>>> @interface.implementer(IContacts)
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>>> checkObject(Contacts(), 'x', Contact())
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>>> checkFactory(Contacts(), 'x', Factory(Contact))
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>>> checkObject(Contacts(), 'x', Buddy())
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... # doctest: +ELLIPSIS
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Traceback (most recent call last):
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>>> checkFactory(Contacts(), 'x', Factory(Buddy))
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The constraints prevent us from moving a container beneath itself (either into
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itself or another folder beneath it):
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>>> container = Container()
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>>> checkObject(container, 'x', container)
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Traceback (most recent call last):
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TypeError: Cannot add an object to itself or its children.
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>>> import zope.location.interfaces
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>>> import zope.interface
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>>> subcontainer = Container()
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>>> zope.interface.directlyProvides(subcontainer,
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... zope.location.interfaces.ILocation)
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>>> subcontainer.__parent__ = container
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>>> checkObject(subcontainer, 'x', container)
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Traceback (most recent call last):
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TypeError: Cannot add an object to itself or its children.
added
removed
src/zope/container/constraints.txt
