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Sat Sep 19 21:22:32 CEST 2015 · http://mail.python.org/pipermail/python-dev/2015-September/141724.html

On 19.09.15 14:03, Eric V. Smith wrote:
> While finishing up the implementation of PEP 498, I realized that the
> PEP has an error. It says that this code:
>
> f'abc{expr1:spec1}{expr2!r:spec2}def{expr3:!s}ghi'
>
> Is equivalent to:
>
> 'abc' + expr1.__format__(spec1) + repr(expr2).__format__(spec2) + 'def'
> + str(expr3).__format__('') + 'ghi'
>
> But that's not correct. The right way to call __format__ is:
>
> type(expr1).__format__(expr1, spec1)
>
> That is, the lookup of __format__ is done on the type, not the instance.
>
> Instead of calling __format__, I've changed the code generator to call
> format(expr1, spec1). As an optimization, I might add special opcodes to
> deal with this and string concatenation, but that's for another day (if
> ever).

Concatenating many strings is not efficient. More efficient way is to 
use string formatting. Why not translate f-string to

     'abc%s%sdef%sghi' % (format(expr1, spec1),
                          format(repr(expr2), spec2), expr3)

or even to

     'abc{:spec1}{!r:spec2}def{!s}ghi'.format(expr1, expr2, expr3)

?

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[Python-Dev] PEP 498 (interpolated f-string) tweak