F-Strings

F-Strings

What it is

F-strings (formatted string literals) provide a concise way to embed expressions inside string literals. Introduced in Python 3.6 (2016), they use f prefix and curly braces for interpolation.

Before this feature

String formatting required verbose methods:

# Old % formatting
name = "Alice"
age = 25
message = "Hello, %s! You are %d years old." % (name, age)

# .format() method
message = "Hello, {}! You are {} years old.".format(name, age)
message = "Hello, {name}! You are {age} years old.".format(name=name, age=age)

After this feature

F-strings provide clean, readable syntax:

# F-string
name = "Alice"
age = 25
message = f"Hello, {name}! You are {age} years old."

# Expressions
x = 10
y = 20
result = f"The sum of {x} and {y} is {x + y}"

# Method calls
text = "python"
formatted = f"Language: {text.upper()}"

# Formatting specifiers
price = 19.99
formatted_price = f"Price: ${price:.2f}"  # "Price: $19.99"

# Multi-line f-strings
user = {"name": "Bob", "role": "admin"}
info = f"""
User Information:
  Name: {user['name']}
  Role: {user['role']}
"""

# Debug feature (Python 3.8+)
value = 42
debug = f"{value=}"  # "value=42"

Why this is better

• Readable: Expressions directly in string
• Concise: Less verbose than .format()
• Fast: Faster than other formatting methods
• Powerful: Can include any Python expression
• Debugging: = specifier shows variable name and value

Key notes / edge cases

• Can’t use backslashes inside {} (use temp variable)
• Can nest f-strings (but readability suffers)
• Raw f-strings: fr"..." or rf"..."
• Evaluated at runtime, not compile time
• Can format numbers, dates, custom objects

# Formatting numbers
value = 1234567.89
formatted = f"{value:,.2f}"  # "1,234,567.89"
percent = f"{0.875:.1%}"     # "87.5%"

# Date formatting
from datetime import datetime
now = datetime.now()
date_str = f"{now:%Y-%m-%d %H:%M:%S}"

# Custom alignment
name = "Alice"
aligned = f"|{name:<10}|"  # left-aligned
aligned = f"|{name:>10}|"  # right-aligned
aligned = f"|{name:^10}|"  # centered

Quick practice

1. Create an f-string that shows “Product: Laptop, Price: $999”

   Answer

   product = "Laptop"
   price = 999
   message = f"Product: {product}, Price: ${price}"

2. Format π (3.14159) to 2 decimal places

   Answer

   import math
   formatted = f"{math.pi:.2f}"  # "3.14"

3. Debug print a variable showing both name and value

   Answer

   count = 42
   print(f"{count=}")  # Prints: count=42