Indexing and Slicing Strings

Every character has a position

Each character in a string has a number called its index. Counting starts at 0, not 1.

For the word PYTHON:

word = "PYTHON"
print(word[0])
print(word[3])
print(word[-1])

Output

P
H
N

Negative indexes count from the end. word[-1] is always the last character – very handy!

Common mistake: word[6] gives an IndexError because the last index of a 6-letter word is 5.

Slicing: cutting out a piece

text[start:stop] gives the characters from start up to, but not including, stop.

Think of it like this: Slicing is like cutting a loaf of bread. [1:4] means: start cutting at slice 1, stop just before slice 4.

word = "PYTHON"
print(word[0:2])
print(word[2:5])

Output

PY
THO

Leave out the start to begin at the start, or leave out the stop to go to the end:

word = "PYTHON"
print(word[:3])
print(word[3:])
print(word[-3:])

Output

PYT
HON
HON

Reversing a string

A third number is the step. A step of -1 walks backwards, which reverses the string:

word = "PYTHON"
print(word[::-1])
print(word[::2])

Output

NOHTYP
PTO

Remember: Strings can't be changed in place. word[0] = "J" gives an error. Instead, build a new string: "J" + word[1:].