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

[Guido van Rossum]

Good catch, Eric.

For those who wonder what the difference is or why it matters, it's what we
do for all dunder operators: the magic method is looked up on the class,
not on the instance. This means you can't e.g. override + on a per-instance
basis by having an instance variable named '__add__' pointing to some
function. That's a rare use case, it's pretty obfuscated, it would slow
down everything, and if you really need that pattern, there are other ways
to do it (you could have a regular instance method __add__ on the class
that looks for an instance variable, e.g. named _add, and call it if it
exists -- otherwise call super().__add__).

--Guido

On Sat, Sep 19, 2015 at 4:03 AM, 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).
>
> I've posted a new version of the code in issue 24965. I'll update the
> PEP itself sometime this weekend.
>
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-- 
--Guido van Rossum (python.org/~guido)
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