Eric V. Smith added the comment:
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]}'
'integer'
>>> '{d[0]}'.format(d=d)
'integer'
>>> f'{d[a]}'
'integer'
>>> '{d[a]}'.format(d=d)
'string'
Note that the exact same expression {d[a]} is evaluated differently by the two
ways to format.
There's a test for this in test_fstring.py.
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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<http://bugs.python.org/issue28308>
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