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

[Eric V. Smith]

On 9/20/2015 8:37 AM, Nick Coghlan wrote:
> On 19 September 2015 at 21:03, Eric V. Smith <eric at trueblade.com> wrote:
>> 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).

Yes, in the current implementation, if you mess with format(), str(),
repr(), or ascii() you can break f-strings. The latter 3 are used to
implement !s, !r, and !a.

I have a plan to change this, by adding one or more opcodes to implement
the formatting and string joining. I'll defer a decision on updating the
PEP until I can establish the feasibility (and desirability) of that
approach.

Eric.