Lesson 37 min read

Strings & Formatting

Words are just arrays of characters wearing a trench coat

What Are Strings?

A string is a sequence of characters — letters, numbers, spaces, emojis, whatever. Under the hood, strings are stored much like a static array of characters. Think of it like a bracelet where each bead is one character. You can look at any bead by its position, but you can't change a bead once it's on the bracelet (strings are immutable).

You can create strings with single quotes 'hello', double quotes "hello", or triple quotes for multi-line text. Python doesn't care which quotes you use — just be consistent.

Creating Strings

# Different ways to create strings
greeting = "Hello, World!"
name = 'Alice'

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

# Strings with quotes inside
dialogue = "She said, 'Let's go!'"
print(dialogue)

# String length
print(len(greeting))

Output

Roses are red,
Violets are blue,
Python is awesome,
And so are you.
She said, 'Let's go!'
13

Indexing & Slicing

Every character in a string has an index (a position number). Python counts from 0, not 1. You can also count backwards using negative indices: -1 is the last character, -2 is second-to-last, and so on.

Slicing lets you grab a chunk of the string using [start:stop:step]. The stop index is NOT included — think of it like a fence: you stop before hitting it.

Indexing & Slicing

word = "PYTHON"
#        P  Y  T  H  O  N
#        0  1  2  3  4  5
#       -6 -5 -4 -3 -2 -1

print(word[0])       # First character
print(word[-1])      # Last character
print(word[1:4])     # Characters 1, 2, 3 (not 4!)
print(word[:3])      # First 3 characters
print(word[3:])      # From index 3 to the end
print(word[::2])     # Every other character
print(word[::-1])    # Reversed!

Output

P
N
YTH
PYT
HON
PTO
NOHTYP

f-Strings: The Best Way to Format

f-strings (formatted string literals) are Python's cleanest way to mix variables into text. Just put an f before the opening quote and use curly braces {} around any expression you want to embed. You can even do math and call functions inside those braces!

f-Strings and Formatting

name = "Luna"
age = 12
height = 4.8333

# Basic f-string
print(f"Hi, I'm {name} and I'm {age} years old!")

# Math inside f-strings
print(f"Next year I'll be {age + 1}")

# Formatting numbers
print(f"Height: {height:.1f} feet")      # 1 decimal place
print(f"Score: {42:05d}")                # Pad with zeros
print(f"Price: ${19.9:.2f}")             # 2 decimal places

# Expressions inside f-strings
items = ["sword", "shield", "potion"]
print(f"You have {len(items)} items")

Output

Hi, I'm Luna and I'm 12 years old!
Next year I'll be 13
Height: 4.8 feet
Score: 00042
Price: $19.90
You have 3 items

Handy String Methods

Strings come with a ton of built-in methods. Since strings are immutable, these methods always return a new string — the original stays unchanged.

Common String Methods

msg = "  Hello, Python World!  "

print(msg.strip())             # Remove leading/trailing spaces
print(msg.lower())             # All lowercase
print(msg.upper())             # ALL CAPS
print(msg.strip().replace("World", "Galaxy"))

# Splitting and joining
csv_line = "apple,banana,cherry"
fruits = csv_line.split(",")   # Split into a list
print(fruits)
print(" & ".join(fruits))      # Join list back into a string

# Searching
story = "The cat sat on the mat"
print(story.count("at"))       # How many times?
print(story.find("sat"))       # Index where it starts
print(story.startswith("The")) # True or False?
print("cat" in story)          # Membership check

Output

Hello, Python World!
  hello, python world!  
  HELLO, PYTHON WORLD!  
Hello, Python Galaxy!
['apple', 'banana', 'cherry']
apple & banana & cherry
3
8
True
True

Note: Strings are immutable — you can't change them in place. When you call msg.upper(), Python creates a brand new string. The original msg is untouched. It's like photocopying a letter and writing on the copy instead of the original.

Quick check

What does "PYTHON"[1:4] return?

Static ArraysData Structure

A row of numbered boxes — fast to grab, fixed in size

→

Trie (Prefix Tree)Data Structure

Insert, search, autocomplete

→

← Operators & Expressions Conditionals →