String Indexing and Slicing

Strings are sequences of characters. Python counts positions starting at 0, so the indices of the characters in "coding" are:

Index	0	1	2	3	4	5
Character	c	o	d	i	n	g

There are 6 characters and the last index is 5 — one less than the length.

Warning

The last valid index is len(s) - 1, not len(s). Accessing s[6] on a six-character string raises an IndexError.

Subscript Notation

Use square brackets to access a single character. Press Try it live to run these and change the index:

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>>> s = "coding"
>>> s[0]
'c'
>>> s[2]
'd'
>>> s[5]
'g'

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The result is always a one-character string.

The subscript can be any expression that evaluates to an integer. Try it live:

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>>> s = "coding"
>>> n = 3
>>> s[n - 1]
'd'

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Negative Indices

Python allows negative indices that count from the right end. Try it live:

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>>> s = "coding"
>>> s[-1]     # last character
'g'
>>> s[-2]     # second from last
'n'
>>> s[-6]     # same as s[0]
'c'

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s[-1] is equivalent to s[len(s) - 1].

Slicing

A slice extracts a substring using the notation s[start:stop]. The result includes characters from index start up to, but not including, index stop. Try it live:

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>>> s = "coding"
>>> s[1:4]
'odi'
>>> s[0:3]
'cod'

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Omitting either end uses the beginning or end of the string. Try it live:

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>>> s = "coding"
>>> s[:3]     # from the start
'cod'
>>> s[3:]     # to the end
'ing'
>>> s[:]      # whole string (copy)
'coding'

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Slices work with negative indices too. Try it live:

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>>> s = "coding"
>>> s[-3:]    # last three characters
'ing'
>>> s[:-2]    # everything except the last two
'codi'

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Step in Slices

An optional third number specifies a step. Try it live:

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>>> s = "abcdefgh"
>>> s[::2]       # every other character
'aceg'
>>> s[::-1]      # reverse the string
'hgfedcba'

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Reversing with [::-1] is an idiomatic Python trick worth remembering.

Indexing Exercise

Predict what each line prints, then run the cell to check your predictions. Experiment with your own slices — try different start, stop, and step values:

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>>> s = "fragment"
>>> k = 3
>>> print(s[1])
r
>>> print(s[k])
g
>>> print(s[2 * k - 2])
m
>>> print(s[-1])
t
>>> print(s[2:5])
agm
>>> print(s[::-1])      # reversed
tnemgarf

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