Python Input & Output: F-Strings in Python

Master Python f-strings with this definitive guide! Learn syntax, formatting tricks, multiline f-strings, expressions, and advanced use cases with clear examples.

What are F-Strings?

F-strings (formatted string literals) are a feature introduced in Python 3.6 that provide a concise and readable way to embed expressions inside string literals. The “f” stands for “formatted” - these strings are evaluated at runtime.

Why Use F-Strings?

1. Readability: Much cleaner syntax compared to older formatting methods
2. Performance: Faster than %-formatting and str.format()
3. Expressiveness: Can include any valid Python expression
4. Less Error-Prone: Fewer moving parts than other formatting methods

Syntax

The basic syntax is simple - prefix your string with ‘f’ or ‘F’ and include expressions in curly braces {}:

f"string text {expression} more text"

Basic Examples for Beginners

1. Simple variable insertion

name = "Alice"
age = 25
print(f"My name is {name} and I'm {age} years old.")

2. Simple calculations

a = 5
b = 10
print(f"The sum of {a} and {b} is {a + b}.")

3. Calling functions

def greet(name):
    return f"Hello, {name}!"

print(f"{greet('Bob')} How are you today?")

4. Formatting numbers

price = 19.99
quantity = 3
print(f"Total: ${price * quantity:.2f}")

Task 1: String Formatting with f-Strings

Instructions

1. Use f-strings (formatted string literals) for the same purpose.

Examples

# Task 5.1: Use f-strings
name = "Ahmad"
age = 30
print(f"My name is {name} and I am {age} years old.")

Python f-Strings Explained: What Does print(f’{a=}’) Do?

The expression print(f”{a=}”) is part of Python’s f-string formatting introduced in Python 3.8.

When you use a=, Python prints both the name of the variable and its value. Essentially, it shows what the variable is and its corresponding value.

Real-Life Examples

1. Database query results

user = {"name": "John", "last_login": "2023-04-15"}
print(f"User {user['name']} last logged in on {user['last_login']}")

2. File processing

filename = "report.pdf"
size = 2.5  # in MB
print(f"File '{filename}' is {size:.1f} MB ({size*1024:.0f} KB)")

3. API responses

response = {"status": "success", "data_count": 42, "time": 0.45}
print(f"API call {response['status']}. Returned {response['data_count']} items in {response['time']}s")

4. Scientific calculations

radius = 5.5
area = 3.14159 * radius**2
print(f"A circle with radius {radius} has area {area:.2f}")

5. Debugging

x = 10
y = 20
print(f"DEBUG: x={x}, y={y}, x*y={x*y}")

Advanced Features

1. Formatting options

value = 123.456789
print(f"Default: {value}")
print(f"2 decimal places: {value:.2f}")
print(f"Scientific notation: {value:.2e}")
print(f"Percentage: {0.25:.1%}")

2. Alignment and padding

item = "Apple"
price = 0.99
print(f"{item:10} - ${price:5.2f}")  # Left-aligned
print(f"{item:>10} - ${price:5.2f}") # Right-aligned

3. Date formatting

from datetime import datetime
today = datetime.now()
print(f"Today is {today:%B %d, %Y}")

F-strings have become the preferred string formatting method in modern Python due to their clarity and efficiency. They’re widely used in logging, debugging, report generation, and anywhere you need to combine variables with text.