[Python-ideas] set.add(x) slower than if x in set:set.add(x)

[Raymond Hettinger]

On Sep 13, 2009, at 11:10 AM, Gerald Britton wrote:
>  Here are some results:
>
> $ python -m timeit -n 1000000  -s 'with open("/usr/share/dict/words")
> as f: s = set(w.strip("\n") for w in f)'  's.add("mother")'
> 1000000 loops, best of 3: 0.292 usec per loop
>
> britton at TheBrittons:~$ python -m timeit -n 1000000  -s 'with
> open("/usr/share/dict/words") as f: s = set(w.strip("\n") for w in f)'
> 'if "mother" not in s:s.add("mother")'
> 1000000 loops, best of 3: 0.185 usec per loop
>
> the second example beats the first by about 36%
>
> Is the timing difference just the cost of the method lookup for s.add,
> or is something else happening that I'm not seeing?

It is the something else you're not seeing ;-)

On the first pass of the 1000000 loops, "mother" gets added.
On the remaining passes the 'if "mother" not in set' test fails
and the set.add() never gets executed.  That latter operation
is a bit more expensive than the contains-test because it
includes the time to lookup and bind the add method.


Raymond