[Python-Dev] Critique of PEP 502 (String Interpolation Redux) (original) (raw)

Guido van Rossum guido at python.org
Sat Sep 5 05:15:48 CEST 2015

The text of the PEP has too much on motivation and rationale: maybe that would be suitable for an informative PEP.

The proposal itself is under-specified.

But the real weakness cannot be fixed by improving the text: it is in the key characteristic of the proposal, which wants to have its cake and eat it, too.

This is initially likable: to recipients who don't know about e-strings they behave just like regular strings; but recipients who do know about them can extract the template and the values to be interpolated and use these to build an alternative string, e.g. to be used as a translation.

However, in order to fulfill the former requirement, the type must inherit from str (it's one of those built-in types for which duck typing doesn't always work). And at the same time this is a real problem, because it's too easy to lose the e-string. For example, any function that operates on strings will operate on e-strings, but the result will in general be a plain str instead of an e-string, which can be confusing for functions that want the e-string. But not inheriting from str causes the reverse problem.

IOW, e-strings don't make a good alternative for f-strings -- they would have to be added as a second string type. And the proliferation of similar but different template string types cannot make anyone happy.

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