Python String Indexing & Slicing

What Is String Indexing?

Indexing means accessing a specific character from a string using its position.

Python treats a string as a sequence of characters. Each character has a position called an index.

word = "Python"

print(word[0])
print(word[1])
print(word[2])

P
y
t

Zero-Based Indexing

Python uses zero-based indexing. This means the first character has index 0, not 1.

word = "Python"

# Index:  0  1  2  3  4  5
# Char:   P  y  t  h  o  n

Therefore:

word = "Python"

print(word[0])  # P
print(word[1])  # y
print(word[2])  # t
print(word[3])  # h
print(word[4])  # o
print(word[5])  # n

Positive Indexing

Positive indexing starts from the beginning of the string and moves from left to right.

language = "Python"

print(language[0])
print(language[3])
print(language[5])

P
h
n

The index increases by one as you move toward the right.

Negative Indexing

Python also allows you to access characters from the end of a string using negative indexes.

word = "Python"

# Positive:  0   1   2   3   4   5
# Character: P   y   t   h   o   n
# Negative: -6  -5  -4  -3  -2  -1

The last character is always at index -1.

word = "Python"

print(word[-1])
print(word[-2])
print(word[-6])

n
o
P

Accessing Individual Characters

You can use an index directly inside square brackets to access a character.

name = "Adnan"

print(name[0])
print(name[2])
print(name[-1])

A
n
n

Indexing returns exactly one character.

IndexError

What happens if you try to access an index that does not exist?

word = "Python"

print(word[10])

You can use len() to determine the number of characters before accessing a specific position.

word = "Python"

index = 4

if index < len(word):
    print(word[index])

What Is String Slicing?

Slicing allows you to extract a portion of a string instead of accessing only one character.

Slicing uses a colon inside square brackets.

text[start:stop]

The start position is included, while the stop position is excluded.

Basic Slice Syntax

Let's extract part of the word Python.

word = "Python"

print(word[0:2])
print(word[2:6])

Py
thon

Notice that index 2 is included in the second slice, but index 6 is not.

Start & Stop Rules

Python slicing follows a simple rule:

word = "Python"

print(word[1:4])

yth

Indexes 1, 2, and 3 are included. Index 4 is excluded.

Omitting the Start Index

If you leave out the start index, Python starts from the beginning of the string.

word = "Python"

print(word[:4])

Pyth

This is equivalent to starting from index 0.

word[:4]

word[0:4]

Omitting the Stop Index

If you leave out the stop index, Python continues until the end of the string.

word = "Python"

print(word[2:])

thon

This means "start at index 2 and continue to the end."

The Step Value

Slicing can also accept a third value called step.

text[start:stop:step]

The step determines how many positions Python moves between selected characters.

word = "Python"

print(word[::2])

Pto

Python selects every second character.

Negative Step & Reversing Strings

A negative step moves through the string from right to left.

One of the most useful Python slicing techniques is:

word = "Python"

reversed_word = word[::-1]

print(reversed_word)

nohtyP

Common Slicing Patterns

Here are several patterns you will use frequently.

word = "Python"

print(word[:3])     # First 3 characters
print(word[3:])     # From index 3 to the end
print(word[:])      # Entire string
print(word[::2])    # Every second character
print(word[::3])    # Every third character
print(word[::-1])   # Reverse the string

Pyt
hon
Python
Pto
Ph
nohtyP

Strings Are Immutable

Python strings are immutable. This means that individual characters cannot be changed directly.

word = "Python"

word[0] = "J"

Instead, create a new string.

word = "Python"

word = "J" + word[1:]

print(word)

Jython