28.03.18 22:05, Paul Moore пише

I can't imagine (non-contrived) code where the fact that a is
formatted before b is evaluated would matter, so I'm fine with option
3.

If formatting a and evaluating b both raise exceptions, the resulting exception depends on the order.

$ ./python -bb
>>> a = b'bytes'
>>> '{}{}'.format(a, b)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'b' is not defined
>>> f'{a}{b}'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
BytesWarning: str() on a bytes instance