[Python-ideas] Briefer string format

[Eric V. Smith]

On 7/21/2015 12:25 AM, Eric V. Smith wrote:
> 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

Oops, that should have been:

$ python3 -mtimeit -s 'a=2; f="{}".format' 'f(a)'
1000000 loops, best of 3: 0.19 usec per loop
$ python3 -mtimeit -s "a=2" "'%s' % a"
10000000 loops, best of 3: 0.138 usec per loop

So, about 40% slower if you can get rid of the .format lookup, which we
can do with f-strings.

Because it's more flexible, .format is just never going to be as fast as
%-formatting. But there's no doubt room for improvement.

The recursive nature of:
f'{o.name:{o.len}}'
will complicate some of the optimizations I've been thinking of.

> 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.

Looks like it's still there. Also for complex!

Eric.