String Basics

Introduction

A string is an immutable sequence of Unicode characters. In Python 3, all strings are Unicode by default.

Creating Strings

s1 = 'Hello'           # Single quotes
s2 = "World"           # Double quotes
s3 = '''Triple single''' # Multi-line
s4 = """Triple double""" # Multi-line

# String repetition
print("Ha" * 3)   # HaHaHa
print(len("Hello"))   # 5

Indexing

s = "Python"

# Positive indexing
print(s[0])   # P
print(s[5])   # n

# Negative indexing
print(s[-1])   # n (last character)
print(s[-6])   # P

Slicing

s = "Hello, World!"

print(s[0:5])    # Hello
print(s[7:12])   # World
print(s[:5])     # Hello (start defaults to 0)
print(s[7:])     # World! (stop defaults to end)
print(s[::2])    # Hlo ol! (every 2nd character)
print(s[::-1])   # Reverse!

Immutability

s = "Hello"
# s[0] = "J"  # TypeError! Strings can't be modified

# Create new string instead
s = "J" + s[1:]   # "Jello"

String Methods

s = "  Hello, World!  "

# Case methods
print(s.upper())          # "  HELLO, WORLD!  "
print(s.lower())          # "  hello, world!  "
print(s.title())          # "  Hello, World!  "
print(s.strip())          # "Hello, World!"

# Finding
print(s.find("World"))    # 7
print(s.find("xyz"))      # -1 (not found)
print(s.count("l"))       # 3

# Replacing
print(s.replace("World", "Python"))  # "Hello, Python!"

# Splitting and Joining
words = s.split(",")      # ['  Hello', ' World!  ']
print("-".join(["a", "b", "c"]))  # a-b-c

# Checking
print("hello123".isalnum())    # True
print("hello".isalpha())       # True
print("123".isdigit())         # True

Why It Matters

Strings are the most common data type in programming — used for text processing, user interaction, data parsing, and more.

Interview Questions

Q1: What is string immutability and why does it matter?

A: Strings cannot be modified after creation. This makes them hashable (dict keys), thread-safe, and allows Python to intern them for efficiency.

Q2: What’s the difference between find() and index()?

A: find() returns -1 if not found; index() raises ValueError.

Practice Exercises

Reverse a string without using [::-1].
Check if a string is a palindrome.
Count character frequency in a string.