[Python-Dev] Mundane dict __setitem__...
David Abrahams
dave@boost-consulting.com
2003年6月03日 13:42:59 -0400
Guido van Rossum <guido@python.org> writes:
>> > Why? You haven't given us much of a clue about your use case.
>>>> I did eventually, in another posting...
>> I have scoured the archives, but have not found your exposition of why
> you would want this. I only found the source of your
> misunderstanding.
>> So I repeat the question (out of honest curiosity only): why would you
> want this?
Sorry, it seems it was a reply to an off-list reply:
-----------
sismex01@hebmex.com writes:
>> Anyone want to tell me it's a bad idea? It seems like the more useful
>> of the two possible behaviors to me.
>>>> I believed hashing was done by value and not necessarily by
> object identity?
Absolutely. The point is that two keys which compare equal may not
neccessarily be strictly equivalent.
I am using a dict which stores the same such object as both the key
and the value of each entry. It basically implements a set which uses
only a part of a value object's data for key comparison and hashing.
A fairly common scenario.
For any given set of key it is important to be able to find the value
that is currently in the set. When I replace the value for that key,
I could write:
d[x] = x
But with no guarantees, the old value object won't be reclaimed since
it is still referenced as the key. To reclaim the reference, I need
del d[x]
d[x] = x
Seems silly to me.
---
BTW, this is not a big deal for me; I don't use Python for work where
speed is critical anyway. A related issue is that I think there
probably ought to be a deldefault method which removes the key if it
exists in the dict and returns the k,v pair.
Regards,
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com