copy-on-write for dict objects?
mwh at python.net
Thu Jan 16 15:25:31 CET 2003
Matthias Oberlaender <matthias.oberlaender at REMOVE.daimlerchrysler.com> writes:
> In <7h3n0m2p77m.fsf at pc150.maths.bris.ac.uk> Michael Hudson wrote:
> > Matthias Oberlaender <matthias.oberlaender at REMOVE.daimlerchrysler.com>
> > > Is there an easy way in Python 2.2.1 to add a copy-on-write mechanism to
> > > subclasses of dict, i.e. without extending/modifying the C-code
> > > implementation? (I guess there is none!)
> > Not sure what you mean. But I doubt it. You need a level of
> > indirection in there, so I don't see how you could do it directly even
> > hacking C.
> > > I don't want to resort to UserDict again, since, at least for my
> > > purposes, it's much slower than direct subclasses of dict (around
> > > 10x)
> > I know that feeling.
> Thanks for all the answers so far (from Andrew, Just and Michael)
> A line of "hacking" I could think of is this:
> class MyDict(dict):
> def __init__(self, d)
> self.sharedDict = d
> self.oldPtr = getTable(self)
> # getTable should be some external function that returns the C pointer to
> # the object's hashtable
> setTable(self, getTable(self.sharedDict))
> # setTable should be some external function that explicitly sets the C
> # pointer to the hashtable
> self.mustCopy = 1
> def _onWrite(self):
> setTable(self, self.oldPtr)
> self.sharedDict = None # give up the reference
> self.mustCopy = 0
> You get the idea? I don't know much about the magic behind the scenes. Maybe
> this is to naive?
Hmm, that could work. Is never going to from Python, though, I'd hazard.
You *could* try this (untested):
__slots__ = ["other"]
class SharedDict(UnSharedDict, UserDict.DictMixin):
__slots__ = ["other"]
def __init__(self, other):
self.other = other
def __getitem__(self, key):
def __setitem__(self, key, value):
super(SharedDict, self).__setitem__(key, value)
self.__class__ = UnSharedDict
def has_key(self, key):
return key in self.other
But you're probably going to find lot's of places that blissfully
ignore the __getitem__ method.
> Why do I want this ?
That wasn't the bit that bothered me :)
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