Accelerate 'string'.format(value, ...) by using formatted string literals #72495

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1. serhiy-storchaka commented on Sep 29, 2016

   

   serhiy-storchaka

   on Sep 29, 2016

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   For now using formatted string literals (PEP-498) is the fastest way of formatting strings.

   $ ./python -m perf timeit -s 'k = "foo"; v = "bar"' -- '"{!s} = {!r}".format(k, v)'
   Median +- std dev: 3.96 us +- 0.17 us
   
   $ ./python -m perf timeit -s 'k = "foo"; v = "bar"' -- 'f"{k!s} = {v!r}"'
   Median +- std dev: 1.09 us +- 0.08 us

   The compiler could translate new-style formatting with literal format string to the equivalent formatted string literal. The code '{!s} = {!r}'.format(k, v) could be translated to

       t1 = k; t2 = v; f'{t1!r} = {t2!s}'; del t1, t2

   or even simpler if k and v are initialized local variables.

   $ ./python -m perf timeit -s 'k = "foo"; v = "bar"' -- 't1 = k; t2 = v; f"{t1!s} = {t2!r}"; del t1, t2'
   Median +- std dev: 1.22 us +- 0.05 us

   This is not easy issue and needs first implementing the AST optimizer.
2. serhiy-storchaka

   added

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   on Sep 29, 2016
3. ericvsmith commented on Sep 29, 2016

   

   ericvsmith

   on Sep 29, 2016

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   One thing to be careful of here is that there's one slight difference between how str.format() and f-strings handle indexing of values. f-strings, of course, use normal Python semantics, but
   str.format() treats indexing by things that don't look like integers as string literals, not variables. It's an unfortunate left-over from the original PEP-3101 specification:

   >>> d = {'a':'string', 0:'integer'}
   >>> a = 0
   >>> f'{d[0]}'
   'integer'
   >>> '{d[0]}'.format(d=d)
   'integer'
   >>> f'{d[a]}'
   'integer'
   >>> '{d[a]}'.format(d=d)
   'string'

   Note that the exact same expression {d[a]} is evaluated differently by the two ways to format.

   There's a test for this in test_fstring.py.

   Someday, I'd like to deprecate this syntax in str.format(). I don't think it could ever be added back in, because it requires either additional named parameters which aren't used as formatting parameters, or it requires global/local lookups (which isn't going to happen).

   i.e., this:
   '{d[a]}'.format(d=d, a=a)
4. serhiy-storchaka

   self-assigned this

   on Dec 25, 2017
5. ezio-melotti

   transferred this issue fromon Apr 10, 2022
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   on Sep 12, 2022
7. StanFromIreland

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   on Aug 13, 2025