str.format(list) with negative index doesn't work in Python

Question

I use a negative index in replacement fields to output a formatted list,but it raises a TypeError.The codes are as follows:

>>> a=[1,2,3]
>>> a[2]
3
>>> a[-1]
3
>>> 'The last:{0[2]}'.format(a)
'The last:3'
>>> 'The last:{0[-1]}'.format(a)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: list indices must be integers, not str

Answer 1

It's what I would call a design glitch in the format string specs. Per the docs,

element_index     ::=  integer | index_string

but, alas, -1 is not "an integer" -- it's an expression. The unary-minus operator doesn't even have particularly high priority, so that for example print(-2**2) emits -4 -- another common issue and arguably a design glitch (the ** operator has higher priority, so the raise-to-power happens first, then the change-sign requested by the lower priority unary -).

Anything in that position in the format string that's not an integer (but, for example, an expression) is treated as a string, to index a dict argument -- for example:

$ python3 -c "print('The last:{0[2+2]}'.format({'2+2': 23}))"
The last:23

Not sure whether this is worth raising an issue in the Python trac, but it's certainly a somewhat surprising behavior:-(.