Python F-Strings

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F-Strings (formatted string literals) are a modern and most efficient way to format strings in Python. Introduced in Python 3.6, f-strings have become the de facto standard for string formatting due to their clean syntax and better performance.

F-String Basic Syntax

F-strings start with the letter f or F before the quote, and Python expressions are written inside curly braces {}:

name = "Budi"
age = 25

# Using f-string
print(f"Hello, my name is {name} and I am {age} years old")
# Output: Hello, my name is Budi and I am 25 years old

# Can also use capital F
print(F"Welcome, {name}!")

Comparison with Old Methods

Before f-strings, there were several ways to format strings:

name = "Andi"
score = 95.5

# Old way 1: Concatenation (+)
print("Name: " + name + ", Score: " + str(score))

# Old way 2: % formatting
print("Name: %s, Score: %.1f" % (name, score))

# Old way 3: .format()
print("Name: {}, Score: {}".format(name, score))

# Modern way: f-string (RECOMMENDED)
print(f"Name: {name}, Score: {score}")

F-strings are easier to read and faster to execute!

Expressions in F-Strings

F-strings can evaluate any Python expression inside the curly braces:

# Mathematical operations
a = 10
b = 5
print(f"Addition: {a + b}")       # Output: Addition: 15
print(f"Multiplication: {a * b}") # Output: Multiplication: 50
print(f"Division: {a / b:.2f}")   # Output: Division: 2.00

# Calling methods
name = "python"
print(f"Uppercase: {name.upper()}")  # Output: Uppercase: PYTHON
print(f"Capitalize: {name.capitalize()}")  # Output: Capitalize: Python

# Calling functions
import math
print(f"Root 16: {math.sqrt(16)}")   # Output: Root 16: 4.0

# Lists and indexing
fruits = ["apple", "orange", "mango"]
print(f"First fruit: {fruits[0]}")    # Output: First fruit: apple

Number Formatting

F-strings have powerful format specifiers to control number display:

# Decimal format
pi = 3.14159265359
print(f"Pi: {pi:.2f}")           # Output: Pi: 3.14
print(f"Pi: {pi:.4f}")           # Output: Pi: 3.1416

# Thousands separator format
population = 1500000
print(f"Population: {population:,}")       # Output: Population: 1,500,000
print(f"Population: {population:_}")       # Output: Population: 1_500_000

# Percentage format
ratio = 0.756
print(f"Percentage: {ratio:.1%}")      # Output: Percentage: 75.6%

# Minimum width format
number = 42
print(f"Number: {number:5}")       # Output: Number:    42 (width 5)
print(f"Number: {number:05}")      # Output: Number: 00042 (padding zero)

# Binary, octal, hexadecimal format
num = 255
print(f"Binary: {num:b}")        # Output: Binary: 11111111
print(f"Octal: {num:o}")         # Output: Octal: 377
print(f"Hex: {num:x}")           # Output: Hex: ff
print(f"Hex (uppercase): {num:X}")  # Output: Hex (uppercase): FF

Format Alignment

text = "Python"

# Left align (default)
print(f"{text:<15}")     # Output: "Python         "

# Right align
print(f"{text:>15}")     # Output: "         Python"

# Center align
print(f"{text:^15}")     # Output: "    Python     "

# With filler characters
print(f"{text:*^15}")    # Output: "****Python*****"
print(f"{text:-<15}")    # Output: "Python---------"

F-String with Dictionary

student = {
    "name": "Siti",
    "id": "12345",
    "gpa": 3.85
}

print(f"Name: {student['name']}, GPA: {student['gpa']}")
# Output: Name: Siti, GPA: 3.85

Debugging with F-String (Python 3.8+)

The = specifier feature is very useful for debugging:

x = 10
y = 20

# Old way
print(f"x = {x}, y = {y}")

# New way with = (Python 3.8+)
print(f"{x=}, {y=}")              # Output: x=10, y=20
print(f"{x + y=}")                # Output: x + y=30
print(f"{x * 2=}")                # Output: x * 2=20

Multiline F-String

name = "Andi"
job = "Developer"
city = "Jakarta"

# Using triple quotes
bio = f"""
User Profile
===============
Name      : {name}
Job       : {job}
City      : {city}
"""

print(bio)

Escaping Curly Braces

If you want to display literal curly braces, use double brackets:

print(f"This is curly braces: {{}}")    # Output: This is curly braces: {}
print(f"Python Set: {{{1, 2, 3}}}")   # Output: Python Set: {1, 2, 3}

Practical Example

# Creating a simple table
products = [
    ("Laptop", 15000000),
    ("Mouse", 250000),
    ("Keyboard", 750000),
]

print(f"{'Product':<15}{'Price':>15}")
print("-" * 30)
for name, price in products:
    print(f"{name:<15}{price:>15,}")

# Output:
# Product                  Price
# ------------------------------
# Laptop            15,000,000
# Mouse                250,000
# Keyboard             750,000