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

[Guido van Rossum]

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.

-- 
--Guido van Rossum (python.org/~guido)
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