Subscriptions
A subscription selects an item of a sequence (string, tuple or list)
or mapping (dictionary) object:
sequence
mapping
string
tuple
list
dictionary
sequenceitem
subscription: primary "[" condition "]"
The primary must evaluate to an object of a sequence or mapping type.
If it is a mapping, the condition must evaluate to an object whose
value is one of the keys of the mapping, and the subscription selects
the value in the mapping that corresponds to that key.
If it is a sequence, the condition must evaluate to a plain integer.
If this value is negative, the length of the sequence is added to it
(so that, e.g. x[-1] selects the last item of x.)
The resulting value must be a nonnegative integer smaller than the
number of items in the sequence, and the subscription selects the item
whose index is that value (counting from zero).
A string's items are characters. A character is not a separate data
type but a string of exactly one character.
stringitem