Why does not the negative indices in python strings result as expected?

I created a string in python : 'HelpA' and labeled it as word.

>>>word='HelpA'

>>>word

'HelpA' #expected string

>>>word[0:4]

'Help' #expected string

While calling the word with negative indices, I am getting the full string instead of partial string as expected.

>>>word[-97:]

'HelpA'  #not the expected string

Its giving the whole string instead of expected string : 'pA' Why does this happen?

Solution

It seems like you expect the negative index to 'wrap'. (97 % len(word) == 2).

It doesn't :)

This example shows you why:

>>> word[-1:]
'A'
>>> word[-3:]
'lpA'
>>> word[-5:]
'HelpA'
>>> word[-10:]
'HelpA'

When the negative index goes beyond the length of the string, it will be truncated. This section of the Python documentation explains explains it like this:

The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j

This code does the equivalent slicing (for strings):

def slice(s, i, j):
    sliced = []
    for k, item in enumerate(s):
        if i <= k and k < j:
            sliced.append(item)
    return ''.join(sliced)

Output:

>>> slice('foo', -100, 2)
'fo'
>>> 'foo'[-100:2]
'fo'

This should make it clear why word[-100:] and word[-99:] gives the same result: all characters in the string have a position or index larger than -100 or -99, since 0 is the first position of the string