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.

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.

# 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

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.

# 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

LocationCopyHook¶

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

zope.copy.interfaces¶

API for zope.copy

exception zope.copy.interfaces.ResumeCopy[source]¶

Don’t use the hook, resume the copy.

This is a special exception, raised from the copy hook to signal copier that it should continue copying the object recursively.

See ICopyHook.__call__ method documentation.

interface zope.copy.interfaces.ICopyHook[source]¶

An adapter to an object that is being copied

__call__(toplevel, register)[source]¶

Given the top-level object that is being copied, return the version of the adapted object that should be used in the new copy.

Raising ResumeCopy means that you are foregoing the hook: the adapted object will continue to be recursively copied as usual.

If you need to have a post-copy actions executed, register a callable with register. This callable must take a single argument: a callable that, given an object from the original, returns the equivalent in the copy.

See README.txt for more explanation.

Getting the Code¶

The main repository for zope.copy is in the Zope Foundation Github repository:

https://github.com/zopefoundation/zope.copy

You can get a read-only checkout from there:

$ git clone https://github.com/zopefoundation/zope.copy.git

or fork it and get a writeable checkout of your fork:

$ git clone git@github.com/jrandom/zope.copy.git

The project also mirrors the trunk from the Github repository as a Bazaar branch on Launchpad:

https://code.launchpad.net/zope.copy

You can branch the trunk from there using Bazaar:

$ bzr branch lp:zope.copy

Working in a virtualenv¶

$ /path/to/virtualenv --no-site-packages /tmp/hack-zope.copy

$ /tmp/hack-zope.copy/bin/python setup.py develop

$ /tmp/hack-zope.copy/bin/python setup.py test -q
...........
----------------------------------------------------------------------
Ran 11 tests in 0.000s

OK

$ /tmp/hack-zope.copy/bin/nosetests
............
----------------------------------------------------------------------
Ran 12 tests in 0.011s

OK

$ .tox/coverage/bin/nosetests --with-coverage
............
Name                   Stmts   Miss  Cover   Missing
----------------------------------------------------
zope.copy                 59      0   100%
zope.copy._compat          6      0   100%
zope.copy.examples         4      0   100%
zope.copy.interfaces       5      0   100%
----------------------------------------------------
TOTAL                     74      0   100%
----------------------------------------------------------------------
Ran 12 tests in 0.062s

OK

$ /tmp/hack-zope.copy/bin/easy_install Sphinx
...
$ cd docs
$ PATH=/tmp/hack-zope.copy/bin:$PATH make html
sphinx-build -b html -d _build/doctrees   . _build/html
...
build succeeded.

Build finished. The HTML pages are in _build/html.

$ PATH=/tmp/hack-zope.copy/bin:$PATH make doctest
sphinx-build -b doctest -d _build/doctrees   . _build/doctest
...
running tests...

Document: narr
--------------
1 items passed all tests:
  93 tests in default
93 tests in 1 items.
93 passed and 0 failed.
Test passed.

Doctest summary
===============
   93 tests
    0 failures in tests
    0 failures in setup code
    0 failures in cleanup code
build succeeded.
Testing of doctests in the sources finished, look at the  \
    results in _build/doctest/output.txt.

Using zc.buildout¶

$ /path/to/python2.6 bootstrap.py
...
Generated script '.../bin/buildout'
$ bin/buildout
Develop: '/home/jrandom/projects/Zope/zope.event/.'
...

$ bin/test --all
Running zope.testing.testrunner.layer.UnitTests tests:
  Set up zope.testing.testrunner.layer.UnitTests in 0.000 seconds.
  Ran 2 tests with 0 failures and 0 errors in 0.000 seconds.
Tearing down left over layers:
  Tear down zope.testing.testrunner.layer.UnitTests in 0.000 seconds.

Using tox¶

$ tox -e py26
GLOB sdist-make: .../zope.copy/setup.py
py26 sdist-reinst: .../zope.copy/.tox/dist/zope.copy-4.0.2dev.zip
py26 runtests: commands[0]
...........
----------------------------------------------------------------------
Ran 11 tests in 0.000s

OK
___________________________________ summary ____________________________________
py26: commands succeeded
congratulations :)

$ tox
GLOB sdist-make: .../zope.copy/setup.py
py26 sdist-reinst: .../zope.copy/.tox/dist/zope.copy-4.0.2dev.zip
py26 runtests: commands[0]
...
Doctest summary
===============
 93 tests
   0 failures in tests
   0 failures in setup code
   0 failures in cleanup code
build succeeded.
___________________________________ summary ____________________________________
py26: commands succeeded
py27: commands succeeded
py32: commands succeeded
py33: commands succeeded
py34: commands succeeded
pypy: commands succeeded
pypy3: commands succeeded
coverage: commands succeeded
docs: commands succeeded
congratulations :)

Contributing to zope.copy¶

$ bzr push lp:~jrandom/zope.copy/cool_feature