[Python-Dev] defaultdict proposal round three (original) (raw)
Steven Bethard steven.bethard at gmail.com
Mon Feb 20 23:58:09 CET 2006
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I wrote:
# I want to do
dd[item] += 1
Guido van Rossum wrote:
You don't need a new feature for that use case; d[k] = d.get(k, 0) + 1 is perfectly fine there and hard to improve upon.
Alex Martelli wrote:
I see d[k]+=1 as a substantial improvement -- conceptually more direct, "I've now seen one more k than I had seen before".
Guido van Rossum wrote:
Yes, I now agree. This means that I'm withdrawing proposal A (new method) and championing only B (a subclass that implements getitem() calling onmissing() and onmissing() defined in that subclass as before, calling defaultfactory unless it's None).
Probably already obvious from my previous post, but FWIW, +1.
Two unaddressed issues:
What module should hold the type? I hope the collections module isn't too controversial.
Should default_factory be an argument to the constructor? The three answers I see:
- "No." I'm not a big fan of this answer. Since the whole point of
creating a defaultdict type is to provide a default, requiring two statements (the constructor call and the default_factory assignment) to initialize such a dictionary seems a little inconvenient.
- "Yes and it should be followed by all the normal dict constructor
arguments." This is okay, but a few errors, like
defaultdict({1:2})will pass silently (until you try to use the dict, of course). - "Yes and it should be the only constructor argument." This is my
favorite mainly because I think it's simple, and I couldn't think of
good examples where I really wanted to do
defaultdict(list, some_dict_or_iterable)ordefaultdict(list, **some_keyword_args). It's also forward compatible if we need to add some of the dict constructor args in later.
STeVe
Grammar am for people who can't think for myself. --- Bucky Katt, Get Fuzzy
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