[Python-Dev] Retrieve an arbitrary element from a set without removing it (original) (raw)

John Arbash Meinel john.arbash.meinel at gmail.com
Fri Oct 23 19:25:48 CEST 2009

Vitor Bosshard wrote:

2009/10/23 Willi Richert <w.richert at gmx.net>:

Hi,

recently I wrote an algorithm, in which very often I had to get an arbitrary element from a set without removing it. Three possibilities came to mind: 1. x = someset.pop() someset.add(x) 2. for x in someset: break 3. x = iter(someset).next()

Of course, the third should be the fastest. It nevertheless goes through all the iterator creation stuff, which costs some time. I wondered, why the builtin set does not provide a more direct and efficient way for retrieving some element without removing it. Is there any reason for this? I imagine something like x = someset.get() I see this as being useful for frozensets as well, where you can't get an arbitrary element easily due to the obvious lack of .pop(). I ran into this recently, when I had a frozenset that I knew had 1 element (it was the difference between 2 other sets), but couldn't get to that element easily (get the pun?)

So in my testing (2) was actually the fastest. I assumed because .next() was a function call overhead, while: for x in some_set: break

Was evaluated inline. It probably still has to call PyObject_GetIter, however it doesn't have to create a stack frame for it.

This is what "timeit" tells me. All runs are of the form: python -m timeit -s "s = set([10])" ...

0.101us "for x in s: break; x" 0.130us "for x in s: pass; x" 0.234us -s "n = next; i = iter" "x = n(i(s)); x" 0.248us "x = next(iter(s)); x" 0.341us "x = iter(s).next(); x"

So 'for x in s: break' is about 2x faster than next(iter(s)) and 3x faster than (iter(s).next()). I was pretty surprised that it was 30% faster than "for x in s: pass". I assume it has something to do with a potential "else:" statement?

Note that all of these are significantly < 1us. So this only matters if it is something you are doing often.

I don't know your specific timings, but I would guess that: for x in s: break

Is actually going to be faster than your s.get()

Primarily because s.get() requires an attribute lookup. I would think your version might be faster for: stat2 = "g = s.get; for i in xrange(100): g()"

However, that is still a function call, which may be treated differently by the interpreter than the for:break loop. I certainly suggest you try it and compare.

John =:->

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