Python Basics — f-Strings

f-Strings

f-strings (formatted string literals) were introduced in Python 3.6 and are the recommended way to format strings. They’re fast, readable, and support inline expressions.

Basic Syntax

Prefix a string with f or F and put expressions inside curly braces {}:

name = "Alice"
age = 25

print(f"Hello, {name}!")          # Hello, Alice!
print(f"{name} is {age} years old.")  # Alice is 25 years old.

Expressions Inside f-Strings

You can put any valid Python expression inside the braces:

# Arithmetic
print(f"2 + 3 = {2 + 3}")          # 2 + 3 = 5

# Function calls
print(f"Name: {'alice'.title()}")   # Name: Alice

# Method calls
items = [1, 2, 3]
print(f"Items: {len(items)}")       # Items: 3

# Conditional expressions
age = 20
print(f"Status: {'adult' if age >= 18 else 'minor'}")
# Status: adult

Formatting Numbers

pi = 3.14159265

# Decimal places
print(f"Pi: {pi:.2f}")       # Pi: 3.14
print(f"Pi: {pi:.4f}")       # Pi: 3.1416

# Comma separator
big_number = 1234567
print(f"Population: {big_number:,}")   # Population: 1,234,567

# Percentage
rate = 0.856
print(f"Success: {rate:.1%}")   # Success: 85.6%

# Zero padding
code = 42
print(f"Code: {code:05}")      # Code: 00042

# Sign display
value = 42
print(f"Positive: {value:+}")   # Positive: +42
print(f"Negative: {-value:+}")  # Negative: -42

Formatting Dates

from datetime import datetime

now = datetime.now()
print(f"Date: {now:%Y-%m-%d}")          # Date: 2025-03-03
print(f"Time: {now:%H:%M:%S}")          # Time: 14:30:45
print(f"Full: {now:%B %d, %Y %I:%M %p}")  # Full: March 03, 2025 02:30 PM

Multi-Line f-Strings

name = "Alice"
age = 25
city = "NYC"

# Using parentheses
message = (
    f"Name: {name}\n"
    f"Age: {age}\n"
    f"City: {city}"
)
print(message)
# Name: Alice
# Age: 25
# City: NYC

# Using triple quotes
report = f"""
Student Report
--------------
Name: {name}
Age: {age}
City: {city}
"""
print(report)

f-String Debugging (= modifier)

Python 3.8+ added the = modifier that shows both the expression and its value:

x = 10
y = 20
print(f"{x = }")          # x = 10
print(f"{y = }")          # y = 20
print(f"{x + y = }")      # x + y = 30
print(f"{x * y = }")      # x * y = 200

name = "Alice"
print(f"{name = }")        # name = 'Alice'
print(f"{len(name) = }")   # len(name) = 5

This is incredibly useful for quick debugging — no need to type print(f"x = {x}").

Alignment in f-Strings

# Left-align (<), right-align (>), center (^)
print(f"{'left':<20}")      # left
print(f"{'right':>20}")     #                right
print(f"{'center':^20}")    #        center

# With fill character
print(f"{'Title':*^30}")    # ************Title*************
print(f"{'Score':->20}")    # ---------------Score
print(f"{42:0>5}")          # 00042

Nested f-Strings

You can nest f-strings in Python 3.12+:

# Works in Python 3.12+
# value = 42
# print(f"{f'{value:05}'}")  # 00042

# For older Python, use a variable
formatted = f"{42:05}"
print(f"Result: {formatted}")  # Result: 00042

Escaping Braces

To include literal curly braces, double them:

print(f"{{Hello}}")     # {Hello}
print(f"x = {10}")      # x = 10
print(f"x = {{10}}")    # x = {10}

# Useful for JSON-like output
name = "Alice"
print(f'{{"name": "{name}"}}')
# {"name": "Alice"}

Performance

f-strings are the fastest string formatting method in Python:

# f-strings are compiled at parse time, making them faster
# than % formatting or str.format()

name = "Alice"

# Fastest
f"Hello, {name}!"

# Slower
"Hello, {}!".format(name)

# Slower
"Hello, %s!" % name

Common Mistakes

Mistake	Example	Fix
Missing f prefix	"{name}"	f"{name}"
Backslash in braces	f"{'\\n'}"	Use a variable
Using quotes inside braces	f"{'it's'}"	Mix quote types

Summary

• f-strings use the f"..." prefix with {expressions}
• Support any Python expression inside braces
• Format numbers with :.2f, :,, :.1%, :05
• Format dates with :%Y-%m-%d, :%H:%M:%S
• Use = modifier for quick debugging (Python 3.8+)
• Use <, >, ^ for alignment
• Double braces {{}} for literal braces
• f-strings are the fastest and recommended formatting method

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