Python f-strings · Clean & Powerful String Formatting

• 1. What are f-strings?
• 2. Why use f-strings? (Comparison)
• 3. Basic variable insertion
• 4. Expressions inside f-strings
• 5. Number formatting
• 6. String formatting
• 7. Multiline f-strings
• 8. Quick reference
• 9. Quick Challenge: Find the f-string
• 10. Exercises
• 11. Common Errors & Solutions
• 12. Check Your Understanding
• 13. Your Progress Tracker

f-strings (formatted string literals) are the easiest way to embed variables inside Python strings. Put an f before the opening quote and use { } around variables.

# Basic f-string syntax
name = "Alice"
age = 25
print(f"My name is {name} and I am {age} years old.")
# Output: My name is Alice and I am 25 years old.

# Why "f"? f stands for "format" or "fast"
# The f tells Python: "This string has expressions inside curly braces"

# Same example with different methods
name = "Alice"
age = 25
city = "New York"

# f-string (BEST - Python 3.6+)
print(f"{name} is {age} years old and lives in {city}")

# .format() method (older)
print("{} is {} years old and lives in {}".format(name, age, city))

# String concatenation (messy)
print(name + " is " + str(age) + " years old and lives in " + city)

# Inserting variables of different types
name = "Bob"
age = 30
height = 1.85
is_student = True

print(f"Name: {name}")
print(f"Age: {age}")
print(f"Height: {height}")
print(f"Student: {is_student}")

# Multiple variables in one string
print(f"{name} is {age} years old, {height}m tall. Student: {is_student}")

You can put any valid Python expression inside the curly braces — not just variable names!

# Arithmetic operations
a = 10
b = 3
print(f"{a} + {b} = {a + b}")
print(f"{a} * {b} = {a * b}")

# Method calls
name = "alice"
print(f"Capitalized: {name.capitalize()}")
print(f"Uppercase: {name.upper()}")

# List and dictionary access
fruits = ["apple", "banana", "cherry"]
print(f"First fruit: {fruits[0]}")

person = {"name": "Alice", "age": 25}
print(f"{person['name']} is {person['age']} years old")

# Conditional expressions (ternary)
score = 85
print(f"You {'passed' if score >= 60 else 'failed'} the exam")

pi = 3.14159265359
price = 24.5
percentage = 0.875
big_number = 1234567

# Decimal places
print(f"pi to 2 decimals: {pi:.2f}")      # 3.14
print(f"Price: ${price:.2f}")              # $24.50

# Percentage
print(f"Score: {percentage:.1%}")          # 87.5%

# Thousands separator
print(f"Big number: {big_number:,}")       # 1,234,567

# Padding and alignment
num = 42
print(f"Right aligned: {num:>5}")   # "   42"
print(f"Zero padding: {num:05d}")   # "00042"

# Hex and binary
print(f"Hex: {255:x}")        # ff
print(f"Binary: {10:b}")       # 1010

# String alignment
word = "Python"
print(f"|{word:10}|")    # |Python    |
print(f"|{word:>10}|")   # |    Python|
print(f"|{word:^10}|")   # |  Python  |

# Truncation
long_word = "HelloWorld"
print(f"{long_word:.5}")    # Hello (truncate to 5 chars)

# Escape curly braces (use double braces)
print(f"{{curly braces}}")    # {curly braces}
print(f"{{{name}}}")          # {Alice}

name = "Alice"
age = 25
city = "New York"

# Using triple quotes
message = f"""
Name: {name}
Age: {age}
City: {city}
"""
print(message)

# Using parentheses to break lines
long_message = (
    f"Hello {name}, "
    f"you are {age} years old. "
    f"We hope you enjoy your stay in {city}!"
)
print(long_message)

# BASIC SYNTAX
f"text {variable} more text"

# NUMBERS
f"{value:.2f}"      # 2 decimal places
f"{value:.0%}"      # percentage
f"{value:,}"        # thousands separator
f"{value:>10}"      # right align (width 10)
f"{value:<10}"      # left align
f"{value:^10}"      # center
f"{value:05d}"      # zero pad to width 5

# STRINGS
f"{text:10}"        # left align width 10
f"{text:>10}"       # right align
f"{text:^10}"       # center
f"{text:.5}"        # truncate to 5 chars

# EXPRESSIONS
f"{a + b}"          # arithmetic
f"{func(x)}"        # function call
f"{list[0]}"        # list access
f"{dict['key']}"    # dict access

# ESCAPE BRACES
f"{{"               # prints {
f"}}"               # prints }

1. What letter do you put before a string to make it an f-string?
2. What symbols do you use to embed variables inside an f-string?
3. How do you format a number to 2 decimal places?
4. How do you add a thousands separator to a number?
5. How do you print literal curly braces in an f-string?