Best way to extract an item from a set of len 1

[Peter Otten]

Alex Martelli wrote:

> Rene Pijlman <reply.in.the.newsgroup at my.address.is.invalid> wrote:
>> Peter Otten:
>> >>>> s = set(["one-and-only"])
>> >>>> item, = s
>    ...
>> >The comma may easily be missed, though.
>> 
>> You could write:
>> 
>>     (item,) = s
>> 
>> But I'm not sure if this introduces additional overhead.
> 
> Naah...:
> 
> helen:~ alex$ python -mtimeit -s's=set([23])' 'x,=s'
> 1000000 loops, best of 3: 0.689 usec per loop
> helen:~ alex$ python -mtimeit -s's=set([23])' '(x,)=s'
> 1000000 loops, best of 3: 0.652 usec per loop
> helen:~ alex$ python -mtimeit -s's=set([23])' '[x]=s'
> 1000000 loops, best of 3: 0.651 usec per loop
> 
> ...much of a muchness.

And that is no coincidence. All three variants are compiled to the same
bytecode:

>>> import dis
>>> def a(): x, = s
...
>>> def b(): (x,) = s
...
>>> def c(): [x] = s
...
>>> dis.dis(a)
  1           0 LOAD_GLOBAL              0 (s)
              3 UNPACK_SEQUENCE          1
              6 STORE_FAST               0 (x)
              9 LOAD_CONST               0 (None)
             12 RETURN_VALUE
>>> dis.dis(b)
  1           0 LOAD_GLOBAL              0 (s)
              3 UNPACK_SEQUENCE          1
              6 STORE_FAST               0 (x)
              9 LOAD_CONST               0 (None)
             12 RETURN_VALUE
>>> dis.dis(c)
  1           0 LOAD_GLOBAL              0 (s)
              3 UNPACK_SEQUENCE          1
              6 STORE_FAST               0 (x)
              9 LOAD_CONST               0 (None)
             12 RETURN_VALUE

Peter