copy-on-write for dict objects?
matthias.oberlaender at REMOVE.daimlerchrysler.com
Thu Jan 16 14:51:51 CET 2003
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:
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 should be some external function that explicitly sets the C
# pointer to the hashtable
self.mustCopy = 1
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?
Why do I want this ? I use dictionaries to implement large sets. In my
framework complementation of sets is allowed and implemented by simply
switching a boolean attribute. Set union, intersection and complementation
can be expressed by using a single type of operator. I call it "neggregation"
(it is analogous to the NOR function). Neggregation is defined to be the
complement of the union of its (arbitray many) arguments. As a special case
we immediately have:
def complement(X): return negg(X).
A bit more subtle, intersection can be expressed like this:
def intersection(A, B): return negg(negg(A), negg(B))
def union(A,B): return negg(negg(A,B))
So, negg(X) with a single argument returns the complement of X. Conceptually,
this requires flipping only a flag. But if this function call also involved
actual copying of lots of data, the above reduction would be fairly
inefficient. Hence I want a copy-on-write.
____ __ _/_/ .
( / / ( / / / /
Matthias Oberlaender, DaimlerChrysler AG, Research Center Ulm
RIC/AP (Machine Perception)
Wilhelm-Runge-Str. 11, P.O. Box 2360, 89013 Ulm, Germany
Phone: +49 731 505 2354 Fax: +49 731 505 4105
Email: matthias.oberlaender at REMOVE.daimlerchrysler.com
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