Simple hooks

First let's examine a simple use. A hook is to support the use case of resetting the state of data that should be changed in a copy -- for instance, a log, or freezing or versioning data. The canonical way to do this is by storing the changable data on a special sub-object of the object that is to be copied. We'll look at a simple case of a subobject that should be converted to None when it is copied -- the way that the zc.freeze copier hook works. Also see the zc.objectlog copier module for a similar example.

So, here is a simple object that stores a boolean on a special object.

>>> class Demo(object):
...     _frozen = None
...     def isFrozen(self):
...         return self._frozen is not None
...     def freeze(self):
...         self._frozen = Data()
...
>>> class Data(object):
...     pass
...

Here's what happens if we copy one of these objects without a copy hook.

>>> original = Demo()
>>> original.isFrozen()
False
>>> original.freeze()
>>> original.isFrozen()
True
>>> import zope.copy
>>> copy = zope.copy.copy(original)
>>> copy is original
False
>>> copy.isFrozen()
True

Now let's make a super-simple copy hook that always returns None, no matter what the top-level object being copied is. We'll register it and make another copy.

>>> import zope.component
>>> import zope.interface
>>> import zope.copy.interfaces
>>> def _factory(obj, register):
...     return None
>>> @zope.component.adapter(Data)
... @zope.interface.implementer(zope.copy.interfaces.ICopyHook)
... def data_copyfactory(obj):
...     return _factory
...

>>> zope.component.provideAdapter(data_copyfactory)
>>> copy2 = zope.copy.copy(original)
>>> copy2 is original
False
>>> copy2.isFrozen()
False

Much better.

Post-copy functions

Now, let's look at the registration function that the hook can use. It is useful for resetting objects within the new copy -- for instance, back references such as __parent__ pointers. This is used concretely in the zc.objectlog.copier module; we will come up with a similar but artificial example here.

Imagine an object with a subobject that is "located" (i.e., zope.location) on the parent and should be replaced whenever the main object is copied.

>>> import zope.location.location
>>> class Subobject(zope.location.location.Location):
...     def __init__(self):
...         self.counter = 0
...     def __call__(self):
...         res = self.counter
...         self.counter += 1
...         return res
...
>>> o = zope.location.location.Location()
>>> s = Subobject()
>>> o.subobject = s
>>> zope.location.location.locate(s, o, 'subobject')
>>> s.__parent__ is o
True
>>> o.subobject()
0
>>> o.subobject()
1
>>> o.subobject()
2

Without an ICopyHook, this will simply duplicate the subobject, with correct new pointers.

>>> c = zope.copy.copy(o)
>>> c.subobject.__parent__ is c
True

Note that the subobject has also copied state.

>>> c.subobject()
3
>>> o.subobject()
3

Our goal will be to make the counters restart when they are copied. We'll do that with a copy hook.

This copy hook is different: it provides an object to replace the old object, but then it needs to set it up further after the copy is made. This is accomplished by registering a callable, reparent here, that sets up the __parent__. The callable is passed a function that can translate something from the original object into the equivalent on the new object. We use this to find the new parent, so we can set it.

>>> import zope.component
>>> import zope.interface
>>> import zope.copy.interfaces
>>> @zope.component.adapter(Subobject)
... @zope.interface.implementer(zope.copy.interfaces.ICopyHook)
... def subobject_copyfactory(original):
...     def factory(obj, register):
...         obj = Subobject()
...         def reparent(translate):
...             obj.__parent__ = translate(original.__parent__)
...         register(reparent)
...         return obj
...     return factory
...
>>> zope.component.provideAdapter(subobject_copyfactory)

Now when we copy, the new subobject will have the correct, revised __parent__, but will be otherwise reset (here, just the counter)

>>> c = zope.copy.copy(o)
>>> c.subobject.__parent__ is c
True
>>> c.subobject()
0
>>> o.subobject()
4

Resuming recursive copy

One thing we didn't examine yet is the use of ResumeCopy exception in the copy hooks. For example, when copying located objects we don't want to copy referenced subobjects that are not located in the object that is being copied. Imagine, we have a content object that has an image object, referenced by the cover attribute, but located in an independent place.

>>> root = zope.location.location.Location()

>>> content = zope.location.location.Location()
>>> zope.location.location.locate(content, root, 'content')

>>> image = zope.location.location.Location()
>>> zope.location.location.locate(image, root, 'image.jpg')

>>> content.cover = image

Without any hooks, the image object will be cloned as well:

>>> new = zope.copy.copy(content)
>>> new.cover is image
False

That's not what we'd expect though, so, let's provide a copy hook to deal with that. The copy hook for this case is provided by zope.location package, but we'll create one from scratch as we want to check out the usage of the ResumeCopy.

>>> @zope.component.adapter(zope.location.interfaces.ILocation)
... @zope.interface.implementer(zope.copy.interfaces.ICopyHook)
... def location_copyfactory(obj):
...     def factory(location, register):
...         if not zope.location.location.inside(obj, location):
...             return obj
...         raise zope.copy.interfaces.ResumeCopy
...     return factory
...
>>> zope.component.provideAdapter(location_copyfactory)

This hook returns objects as they are if they are not located inside object that's being copied, or raises ResumeCopy to signal that the recursive copy should be continued and used for the object.

>>> new = zope.copy.copy(content)
>>> new.cover is image
True

Much better :-)

clone vs copy

As we stated before, there's two functions that is used for copying objects. The clone - that does the job, and its wrapper, copy that calls clone and then clears copy's __parent__ and __name__ attribute values.

Let's create a location object with __name__ and __parent__ set.

>>> root = zope.location.location.Location()
>>> folder = zope.location.location.Location()
>>> folder.__name__ = 'files'
>>> folder.__parent__ = root

The clone function will leave those attributes as is. Note that the referenced __parent__ won't be cloned, as we registered a hook for locations in the previous section.

>>> folder_clone = zope.copy.clone(folder)
>>> folder_clone.__parent__ is root
True
>>> folder_clone.__name__ == 'files'
True

However, the copy function will reset those attributes to None, as we will probably want to place our object into another container with another name.

>>> folder_clone = zope.copy.copy(folder)
>>> folder_clone.__parent__ is None
True
>>> folder_clone.__name__ is None
True

Notice, that if your object doesn't have __parent__ and __name__ attributes at all, or these attributes could'nt be got or set because of some protections (as with zope.security's proxies, for example), you still can use the copy function, because it works for objects that don't have those attributes.

It won't set them if original object doesn't have them:

>>> class Something(object):
...     pass

>>> s = Something()
>>> s_copy = zope.copy.copy(s)
>>> s_copy.__parent__
Traceback (most recent call last):
...
AttributeError: ...
>>> s_copy.__name__
Traceback (most recent call last):
...
AttributeError: ...

And it won't fail if original object has them but doesn't allow to set them.

>>> root = object()
>>> class Something(object):
...
...     @apply
...     def __name__():
...         def fget(self):
...             return 'something'
...         def fset(self, value):
...             raise AttributeError
...         return property(fget, fset)
...
...     @apply
...     def __parent__():
...         def fget(self):
...             return root
...         def fset(self, value):
...             raise AttributeError
...         return property(fget, fset)

>>> s = Something()
>>> s_copy = zope.copy.copy(s)
>>> s_copy.__parent__ is root
True
>>> s_copy.__name__ == 'something'
True

3.5.1 (unreleased)

• Include tests of the LocationCopyHook from zope.location.

3.5.0 (2009-02-09)

• Initial release. The functionality was extracted from zc.copy to provide a generic object copying mechanism with minimal dependencies.