[Python-Dev] Subtle difference between f-strings and str.format() (original) (raw)

Stefan Behnel stefan_ml at behnel.de
Sat Mar 31 04:17:37 EDT 2018

Serhiy Storchaka schrieb am 28.03.2018 um 17:27:

There is a subtle semantic difference between str.format() and "equivalent" f-string.

'{}{}'.format(a, b) f'{a}{b}' In the former case b is evaluated before formatting a. This is equivalent to t1 = a t2 = b t3 = format(t1) t4 = format(t2) r = t3 + t4 In the latter case a is formatted before evaluating b. This is equivalent to t1 = a t2 = format(t1) t3 = b t4 = format(t3) r = t2 + t4 In most cases this doesn't matter, but when implement the optimization that transforms the former expression to the the latter one ([1], [2]) we have to make a decision what to do with this difference. [1] https://bugs.python.org/issue28307 [2] https://bugs.python.org/issue28308

Just for the record, I implemented the translation-time transformation described in [1] (i.e. '%' string formatting with a tuple) in Cython 0.28 (released mid of March, see [3]), and it has the same problem of changing the behaviour. I was aware of this at the time I implemented it, but decided to postpone the fix of evaluating the arguments before formatting them, as I considered it low priority compared to the faster execution. I expect failures during formatting to be rare in real-world code, where string building tends to be at the end of a processing step that should catch many value problems already. Rare, but not impossible.

I do consider this change in behaviour a bug that should be fixed, and I would also consider it a bug in CPython if it was added there.

Stefan

[3] https://github.com/cython/cython/blob/de618c0141ae818e7a4c35d46256d98e6b6dba53/Cython/Compiler/Optimize.py#L4261

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