[Python-3000] Python-3000 Digest, Vol 2, Issue 151 (original) (raw)

Guido van Rossum guido at python.org
Thu Apr 27 06:26:40 CEST 2006

On 4/26/06, Antoine Pitrou <solipsis at pitrou.net> wrote:

1) sets are most often built dynamically rather than statically (in my own experience)

The same is true to the same extent for lists and dicts.

And yet there are many uses for list and dict literals.

I've seen a lot of list and tuple literals that should have been sets, because the main usage was to test whether some value was "in" the list/tuple or not. That's an O(N) operation -- fortunately it's pretty fast for tiny N.

2) set([1,2,3]) makes little sense anyway, since it probably isn't significantly more efficient than [1,2,3]

Here's a benchmark:

guido at pythonic:guido$ python2.4 -m timeit -s 'x = (1,2,3)' '(4 in x)' 1000000 loops, best of 3: 0.243 usec per loop guido at pythonic:guido$ python2.4 -m timeit -s 'x = set((1,2,3))' '(4 in x)' 10000000 loops, best of 3: 0.147 usec per loop

The tuple version is slightly faster for (1 in x); but it's slower for (2 in x), slower still for (3 in x). You can easily extrapolate from this.

The real benefit of sets is when you have at least tens or hundreds of elements; in that case you won't use a literal to build the set.

Have a look at keyword.py in the stdlib.

Another remark is that I often use sets to hold my own objects rather than simple values (ints or strings). In a sense, I use sets as iterable containers where arbitrary remove() is fast.

Sure. That's not an argument against set literals.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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