Python Input & Output: F-Strings in Python

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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?
Why Use F-Strings?
Basic Examples for Beginners
Advanced Features
Real-Life Examples

1. 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.

2. Why Use F-Strings?

Readability: Much cleaner syntax compared to older formatting methods
Performance: Faster than %-formatting and str.format()
Expressiveness: Can include any valid Python expression
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"

3. 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

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.")

4. 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}")

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

for more details, see Python f-Strings with = and Format Specifiers (Debugging Made Easy)

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.

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Python Input & Output: F-Strings in Python

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