Copying persistent objects¶
This package provides a pluggable way to copy persistent objects. It was once extracted from the zc.copy package to contain much less dependencies. In fact, we only depend on zope.interface to provide pluggability.
The package provides a clone() function that does the object cloning and the copy() wrapper that sets __parent__ and __name__ attributes of object’s copy to None. This is useful when working with Zope’s located objects (see zope.location package). The copy() function actually calls the clone() function, so we’ll use the first one in the examples below. We’ll also look a bit at their differences in the end of this document.
The clone() function (and thus the copy() function that wraps it) uses pickling to copy the object and all its subobjects recursively. As each object and subobject is pickled, the function tries to adapt it to zope.copy.interfaces.ICopyHook. If a copy hook is found, the recursive copy is halted. The hook is called with two values: the main, top-level object that is being copied; and a callable that supports registering functions to be called after the copy is made. The copy hook should return the exact object or subobject that should be used at this point in the copy, or raise zope.copy.interfaces.ResumeCopy exception to resume copying the object or subobject recursively after all.
Note that we use zope’s component architecture provided by the zope.component package in this document, but the zope.copy package itself doesn’t use or depend on it, so you can provide another adaptation mechanism as described in zope.interface‘s adapter documentation.
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.
# zope.copy.examples.Demo class Demo(object): #pragma NO COVER _frozen = None def isFrozen(self): return self._frozen is not None def freeze(self): self._frozen = Data()
# zope.copy.examples.Data class Data(object): #pragma NO COVER pass
Here’s what happens if we copy one of these objects without a copy hook.
>>> from zope.copy.examples import Demo, Data >>> 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
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.
# zope.copy.examples.Subobject class Subobject(zope.location.location.Location): #pragma NO COVER def __init__(self): self.counter = 0 def __call__(self): res = self.counter self.counter += 1 return res
>>> import zope.location.location >>> from zope.copy.examples import Subobject >>> 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:
# zope.copy.examples.Something class Something(object): #pragma NO COVER pass
>>> from zope.copy.examples import Something >>> 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.
# zope.copy.examples.Other class Other(object): #pragma NO COVER @_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)
>>> from zope.copy.examples import Other >>> from zope.copy.examples import root >>> s = Other() >>> s_copy = zope.copy.copy(s) >>> s_copy.__parent__ is root True >>> s_copy.__name__ == 'something' True
The location copy hook is defined in zope.location but only activated if this package is installed.
It’s job is to allow copying referenced objects that are not located inside object that’s being copied.
To see the problem, imagine we want to copy an ILocation object that contains an attribute-based reference to another ILocation object and the referenced object is not contained inside object being copied.
Without this hook, the referenced object will be cloned:
>>> from zope.component.globalregistry import base >>> base.__init__('base') # blow away previous registrations >>> from zope.location.location import Location, locate >>> root = Location() >>> page = Location() >>> locate(page, root, 'page') >>> image = Location() >>> locate(page, root, 'image') >>> page.thumbnail = image >>> from zope.copy import copy >>> page_copy = copy(page) >>> page_copy.thumbnail is image False
But if we will provide a hook, the attribute will point to the original object as we might want.
>>> from zope.component import provideAdapter >>> from zope.location.pickling import LocationCopyHook >>> from zope.location.interfaces import ILocation >>> provideAdapter(LocationCopyHook, (ILocation,)) >>> from zope.copy import copy >>> page_copy = copy(page) >>> page_copy.thumbnail is image True