[Python-ideas] Briefer string format

[Eric V. Smith]

On 7/21/2015 12:02 AM, Alexander Belopolsky wrote:
> 
> On Mon, Jul 20, 2015 at 11:35 PM, Tim Peters <tim.peters at gmail.com
> <mailto:tim.peters at gmail.com>> wrote:
> 
>     >>> dis(f)
>       2           0 LOAD_CONST               3 ('2')
>                   3 RETURN_VALUE
> 
>     That is, the peephole optimizer got rid of "%d" % 2 entirely,
>     replacing it with the string constant "2".  So, in all, it's more
>     surprising that it takes so long to load a constant ;-)
> 
> 
> Hmm.  I stand corrected:
> 
> $ python3 -mtimeit -s "a=2" "'%s' % a"
> 10000000 loops, best of 3: 0.124 usec per loop
> $ python3 -mtimeit -s "a=2" "'{}'.format(a)"
> 1000000 loops, best of 3: 0.215 usec per loop
> 
> it is 2x rather than 20x speed difference.

The last time I looked at this, the performance difference was the
lookup of "format" on a string object. Although maybe that's not true,
and the problem is really function call overhead:

$ python3 -mtimeit -s 'a=2' 'f="{}".format' 'f(a)'
1000000 loops, best of 3: 0.227 usec per loop
$ python3 -mtimeit -s "a=2" "'%s' % a"
10000000 loops, best of 3: 0.126 usec per loop

There is (or was) a special case for formatting str, int, and float to
bypass the .__format__ lookup. I haven't looked at it since the PEP 393
work.

Eric.