String indexing - Why S[0][0] works and S[1][1] fails?

Question

Suppose I create a string:

>>> S = "spam"

Now I index it as follows:

>>> S[0][0][0][0][0]

I get output as:

>>> 's'

But when i index it as:

>>> S[1][1][1][1][1]

I get output as:

Traceback (most recent call last):
File "<pyshell#125>", line 1, in <module>
L[1][1][1][1][1]
IndexError: string index out of range

Why is the output not 'p'?

Why also it is working for S[0][0] or S[0][0][0] or S[0][0][0][0] and not for S[1][1] or S[1][1][1] or S[1][1][1][1]?

Answer 1

The answer is that S[0] gives you a string of length 1, which thus necessarily has a character at index 0. S[1] also gives you a string of length 1, but it necessarily does not have a character at index 1. See below:

>>> S = "spam"
>>> S[0]
's'
>>> S[0][0]
's'
>>> S[1]
'p'
>>> S[1][0]
'p'
>>> S[1][1]
Traceback (most recent call last):
  File "<pyshell#20>", line 1, in <module>
    S[1][1]
IndexError: string index out of range

Answer 2

The first index ([0]) of any string is its first character. Since this results in a one-character string, the first index of that string is the first character, which is itself. You can do [0] as much as you want and stay with the same character.

The second index ([1]), however, only exists for a string with at least two characters. If you've already indexed a string to produce a single-character string, [1] will not work.

>>> a = 'abcd'
>>> a[0]
'a'
>>> a[0][0]
'a'
>>> a[1]
'b'
>>> a[1][0][0]
'b'
>>> a[1][1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range