Accessing Characters in a String

Every character inside a string occupies a numbered position, and the technique of reaching a character by its position number is called indexing. The index is written inside square brackets [ ] immediately after the string (or the variable holding it), and it tells Python exactly which character to fetch.

One fact worth fixing in mind first: Python has no separate character data type. A single character is simply a string whose length is one — so indexing a string gives you back another (one-character) string.

Positive indexing — counting from the left

Counting from the left, the first character has index 0 and the last character has index n-1, where n is the length of the string. Asking for any index outside this range raises an IndexError.

>>> s = 'Hello World!'   # 12 characters, indices 0 to 11
>>> s[0]                 # first character
'H'
>>> s[6]                 # seventh character (index 6)
'W'
>>> s[11]                # last character (index 11 = 12-1)
'!'
>>> s[15]                # beyond index 11 -> error
IndexError: string index out of range

The index must be an integer

The index need not be a bare number — it can be any expression, built from variables and operators, provided the expression evaluates to an integer. A fractional index is rejected with a TypeError:

>>> s[2 + 4]     # expression evaluates to 6 -> the 7th character
'W'
>>> s[1.5]       # not an integer -> error
TypeError: string indices must be integers

Negative indexing — counting from the right

Python also permits negative index values, used when it is more natural to count characters from right to left. Counting from the right-hand side, the rightmost character has index -1, the next one -2, and so on, until the leftmost character has index -n (n being the string's length).

>>> s[-1]      # first character from the right
'!'
>>> s[-12]     # twelfth character from the right = leftmost
'H'

For the string 'Hello World!' the two numbering systems line up like this:

So every character can be named in two equivalent ways — s[6] and s[-6] both give 'W'.

Finding the length: the len() function

Python's inbuilt function len() returns the length of the string passed to it — the total count of characters, spaces included. For 'Hello World!' the length is 12.

>>> len(s)
12 …