Either leading sign, '+' or '-', cause string interpretation, so I think 'unsigned integer' should be the term in the doc.

>>> '{0[-1]}'.format({'-1': 'neg int key'})
'neg int key'
>>> '{0[+1]}'.format({'+1': 'neg int key'})
'neg int key'
>>> '{0[+1]}'.format([1,2,3])
Traceback (most recent call last):
  File "<pyshell#16>", line 1, in <module>
    '{0[+1]}'.format([1,2,3])
TypeError: list indices must be integers, not str