[Python-Dev] optimization required: .format() is much slower than %

[Antoine Pitrou]

Simon Cross <hodgestar <at> gmail.com> writes:
> My tests show that the old-style % formatting is much faster when the
> final string is 20 characters or less:
> 
> $ ./python -m timeit "'....|....|....|...%s' % '12'"
> 10000000 loops, best of 3: 0.0764 usec per loop


You are the victim of a constant-folding optimization:

$ ./python -m timeit             "'....|....|....|...%s' % '12'"
10000000 loops, best of 3: 0.0926 usec per loop
$ ./python -m timeit -s "s='12'" "'....|....|....|...%s' % s"
1000000 loops, best of 3: 0.525 usec per loop


>>> def f(): return '....|....|....|...%s' % '12'
... 
>>> dis.dis(f)
  1           0 LOAD_CONST               3 ('....|....|....|...12') 
              3 RETURN_VALUE         


cheers

Antoine.