Strings & Formatting

String Fundamentals

Strings are one of the most frequently used data types in any program. Every time you display a message, read a file, process user input, or build a URL, you're working with strings. In Python, strings are immutable sequences of Unicode characters — meaning once created, they can't be modified. Every string method returns a new string.

Creating Strings

Python offers several ways to create strings. Single quotes and double quotes are identical. Triple quotes let you span multiple lines. Raw strings ignore escape characters:

# Single and double quotes are identical
s1 = 'hello'
s2 = "hello"
print(s1 == s2)  # True

# Use the other style to include quotes
message = "It's a beautiful day"
html = '<div class="box">content</div>'

# Triple quotes for multiline
poem = """Roses are red,
Violets are blue,
Python is awesome,
And so are you."""
print(poem)

# Raw strings — no escape processing (great for regex and paths)
path = r"C:\Users\name\documents"
print(path)  # Backslashes are literal, not escape chars

True
Roses are red,
Violets are blue,
Python is awesome,
And so are you.
C:\Users\name\documents

String Indexing and Slicing

Strings are sequences, so you can access individual characters by index and extract substrings with slicing. Remember: indexing starts at 0, and negative indices count from the end:

s = "Python"

# Indexing
print(s[0])     # P (first)
print(s[-1])    # n (last)
print(s[2])     # t (third)

# Slicing: s[start:stop:step]
print(s[1:4])   # yth (index 1,2,3)
print(s[:3])    # Pyt (first 3)
print(s[3:])    # hon (from index 3 on)
print(s[::-1])  # nohtyP (reversed!)
print(s[::2])   # Pto (every other char)

P
n
t
yth
Pyt
hon
nohtyP
Pto

f-strings: Modern String Formatting

f-strings (formatted string literals, Python 3.6+) are the clearest and fastest way to embed expressions in strings. Prefix the string with f and put expressions inside curly braces:

name = "Alice"
age = 30
balance = 1234.5678

# Basic embedding
print(f"{name} is {age} years old")

# Expressions inside braces
print(f"Next year: {age + 1}")
print(f"Name uppercased: {name.upper()}")

# Format specifiers
print(f"Balance: ${balance:,.2f}")    # Comma + 2 decimals
print(f"Percentage: {0.856:.1%}")     # As percentage
print(f"{name:>15}")                  # Right-align in 15 chars
print(f"{name:*^15}")                 # Center with * padding
print(f"{'yes' if age > 18 else 'no'}")  # Inline conditional

# Debugging (Python 3.8+)
x = 42
print(f"{x = }")  # Prints: x = 42

Alice is 30 years old
Next year: 31
Name uppercased: ALICE
Balance: $1,234.57
Percentage: 85.6%
          Alice
*****Alice*****
yes
x = 42

Essential String Methods

Python strings come with dozens of built-in methods. Here are the ones you'll use most often. Remember: all string methods return new strings because strings are immutable.

s = "  Hello, World!  "

# Cleaning
print(s.strip())          # "Hello, World!" (remove whitespace)
print(s.lstrip())         # "Hello, World!  " (left only)
print(s.rstrip())         # "  Hello, World!" (right only)

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

# Searching
print("World" in s)            # True
print(s.find("World"))         # 9 (index, -1 if not found)
print(s.count("l"))            # 3

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

# Splitting and joining
words = "one,two,three".split(",")
print(words)                # ['one', 'two', 'three']
print(" | ".join(words))    # "one | two | three"

# Checking
print("hello123".isalnum())     # True (letters + digits)
print("hello".isalpha())        # True (letters only)
print("12345".isdigit())        # True (digits only)
print("hello".startswith("he")) # True
print("hello".endswith("lo"))   # True

Hello, World!
True
9
3
Hello, Python!
['one', 'two', 'three']
one | two | three
True
True
True
True
True

result = ""
for word in words:
    result += word + " "  # Creates a NEW string every iteration!

result = " ".join(words)  # O(n) — builds the string once