Author eric.smith
Recipients eric.smith, serhiy.storchaka
Date 2016-09-29.09:09:54
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One thing to be careful of here is that there's one slight difference between how str.format() and f-strings handle indexing of values. f-strings, of course, use normal Python semantics, but 
str.format() treats indexing by things that don't look like integers as string literals, not variables. It's an unfortunate left-over from the original PEP-3101 specification:

>>> d = {'a':'string', 0:'integer'}
>>> a = 0
>>> f'{d[0]}'
>>> '{d[0]}'.format(d=d)
>>> f'{d[a]}'
>>> '{d[a]}'.format(d=d)

Note that the exact same expression {d[a]} is evaluated differently by the two ways to format.

There's a test for this in

Someday, I'd like to deprecate this syntax in str.format(). I don't think it could ever be added back in, because it requires either additional named parameters which aren't used as formatting parameters, or it requires global/local lookups (which isn't going to happen).

i.e., this:
'{d[a]}'.format(d=d, a=a)
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