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

[Nick Coghlan]

On 19 September 2015 at 21:03, Eric V. Smith <eric at trueblade.com> 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).

Does this mean overriding format at the module level or in builtins
will affect the way f-strings are evaluated at runtime? (I don't have
a strong preference one way or the other, but I think the PEP should
be explicit as to the expected behaviour rather than leaving it as
implementation defined).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia