[Python-Dev] defaultdict proposal round three (original) (raw)

Tim Peters tim.peters at gmail.com
Tue Feb 21 05:44:20 CET 2006

[Guido]

... What's the practical use case for not wanting contains() to function?

I don't know. I have practical use cases for wanting contains() to function, but there's been no call for those. For an example, think of any real use ;-)

For example, I often use dicts to represent multisets, where a key maps to a strictly positive count of the number of times that key appears in the multiset. A default of 0 is the right thing to return for a key not in the multiset, so that M[k] += 1 works to add another k to multiset M regardless of whether k was already present.

I sure hope I can implement multiset intersection as, e.g.,

def minter(a, b): if len(b) < len(a): # make a the smaller, and iterate over it a, b = b, a result = defaultdict defaulting to 0, however that's spelled for k in a: if k in b: result[k] = min(a[k], b[k]) return result

Replacing the loop nest with:

for k in a:
    result[k] = min(a[k], b[k])

would be semantically correct so far as it goes, but pragmatically wrong: I maintain my "strictly positive count" invariant because consuming RAM to hold elements "that aren't there" can be a pragmatic disaster. (When k is in a but not in b, I don't want k to be stored in result)

I have other examples, but they come so easily it's better to leave that an exercise for the reader.

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