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

Serhiy Storchaka storchaka at gmail.com
Sun Sep 20 17:15:23 CEST 2015

On 20.09.15 16:51, Eric V. Smith wrote:

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.

I propose to add internal builting formatter type. Instances should be marshallable and callable. The code generated for f-strings should just load formatter constant and call it with arguments. The formatter builds resulting string by concatenating literal strings and results of formatting arguments with specified specifications.

Later we could change compiler (just peephole optimizer?) to replace literal_string.format(*args) and literal_string % args with calling precompiled formatter.

Later we could rewrite str.format, str.mod and re.sub to create temporary formatter object and call it.

Later we could expose public API for creating formatter object. It can be used by third-party template engines.

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