String Formatting & F-Strings

Table of Contents

• What is String Formatting?
• The Old Way: % Formatting
  • Basic % formatting
  • Format specifiers
  • Multiple values
• The .format() Method
  • Basic usage
  • Positional and keyword arguments
  • Format specifications
• F-Strings (The Modern Way)
  • Basic f-string syntax
  • Expressions inside f-strings
  • Formatting numbers
  • Alignment and padding
  • Calling methods inside f-strings
• Which One Should You Use?
• Try It Yourself

What is String Formatting?

You have variables with values, and you want to build a string that includes those values in a clean, readable way. That is string formatting.

Instead of this messy approach:

name = "Alex"
age = 25
print("My name is", name, "and I am", age, "years old")

You can do this:

print(f"My name is {name} and I am {age} years old")

Much cleaner, right? Python gives you several ways to format strings. Let us look at all of them, from oldest to newest.

The Old Way: % Formatting

This is the original Python way, borrowed from the C programming language. You will see it in old code, but you probably won't use it much yourself.

Basic % formatting

You create a string with %s placeholders (or other specifiers), and then provide the values after a % symbol:

name = "Alice"
print("Hello, %s!" % name)         # Hello, Alice!

age = 25
print("%s is %s years old" % (name, age))   # Alice is 25 years old

Format specifiers

name = "Bob"
score = 87.45678

print("Name: %s" % name)           # Name: Bob
print("Score: %d" % score)         # Score: 87  (truncates decimal!)
print("Score: %f" % score)         # Score: 87.456780
print("Score: %.2f" % score)       # Score: 87.46  (rounded to 2 decimals)

Multiple values

Use parentheses (a tuple) when you have more than one value:

name = "Charlie"
age = 30
score = 92.5
print("%s is %d years old with a score of %.1f" % (name, age, score))
# Charlie is 30 years old with a score of 92.5

The % formatting works, but it gets messy with many variables. The order matters, and it is easy to make mistakes.

The .format() Method

Python 3 introduced a nicer way — the .format() method on strings. You use curly braces {} as placeholders.

Basic usage

name = "Diana"
age = 28
print("My name is {} and I am {} years old".format(name, age))
# My name is Diana and I am 28 years old

Positional and keyword arguments

You can use numbers (positions) or names inside the braces:

# Positional — order matters
print("{0} is {1} years old. {0} loves Python.".format("Eve", 22))
# Eve is 22 years old. Eve loves Python.

# Keyword — names matter, order doesn't
print("{name} is {age} years old.".format(name="Frank", age=35))
# Frank is 35 years old.

Format specifications

You can control number formatting, alignment, and more inside the curly braces by adding a colon : after the position/name:

pi = 3.14159265

print("{:.2f}".format(pi))        # 3.14  (2 decimal places)
print("{:.3f}".format(pi))        # 3.142 (3 decimal places)
print("{:10.2f}".format(pi))      # "      3.14"  (total width 10)
print("{:<10.2f}".format(pi))     # "3.14      "  (left-aligned)
print("{:>10.2f}".format(pi))     # "      3.14"  (right-aligned)
print("{:^10.2f}".format(pi))     # "  3.14    "  (center-aligned)

Let us see alignment with text:

print("{:<10} {:>10}".format("Left", "Right"))
# Left            Right

print("{:^20}".format("CENTER"))
#       CENTER

F-Strings (The Modern Way)

Python 3.6 introduced f-strings (formatted string literals), and honestly, they are the best. You put an f before the opening quote, then put any Python expression inside {}.

Basic f-string syntax

name = "Grace"
age = 32
print(f"My name is {name} and I am {age} years old")
# My name is Grace and I am 32 years old

Notice how much cleaner this is than .format() or %. The variable names go directly inside the braces.

Expressions inside f-strings

You are not limited to just variable names. You can put any Python expression inside {}:

a = 10
b = 3
print(f"{a} + {b} = {a + b}")                 # 10 + 3 = 13
print(f"{a} divided by {b} is {a / b:.2f}")   # 10 divided by 3 is 3.33

name = "Alice"
print(f"Hello, {name.upper()}")                # Hello, ALICE

items = 3
price = 4.99
print(f"Total: ${items * price:.2f}")          # Total: $14.97

You can even use conditional expressions:

score = 85
print(f"Result: {'Pass' if score >= 60 else 'Fail'}")
# Result: Pass

Formatting numbers

F-strings use the same format specifiers as .format(), just inside the braces after a colon:

pi = 3.14159265
print(f"Pi to 2 decimals: {pi:.2f}")           # Pi to 2 decimals: 3.14
print(f"Pi to 4 decimals: {pi:.4f}")           # Pi to 4 decimals: 3.1416

big_number = 1234567
print(f"With commas: {big_number:,}")          # With commas: 1,234,567

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

Here is what the format specifier inside {variable:specifier} means:

value = 12.3456
print(f"{value:.1f}")    # 12.3   — 1 digit after decimal
print(f"{value:.2f}")    # 12.35  — 2 digits after decimal
print(f"{value:10.2f}")  # "     12.35" — total width 10, 2 decimals

Alignment and padding

name = "Python"

print(f"{name:>10}")    # "    Python"  — right align
print(f"{name:<10}")    # "Python    "  — left align
print(f"{name:^10}")    # "  Python  "  — center
print(f"{name:*^20}")   # "*******Python********"  — center with * padding

Calling methods inside f-strings

text = "hello world"
print(f"{text.upper()}")          # HELLO WORLD
print(f"{text.title()}")          # Hello World
print(f"{len(text)} characters")  # 11 characters

price = 19.99
print(f"${price:.2f}")            # $19.99

Multi-line f-strings

name = "Isaac"
age = 29
city = "Berlin"

info = (
    f"Name: {name}\n"
    f"Age: {age}\n"
    f"City: {city}"
)
print(info)
# Name: Isaac
# Age: 29
# City: Berlin

Which One Should You Use?

Rule of thumb: Use f-strings. They are the modern Python way and almost always the best choice.

Try It Yourself

Copy and run this whole block in your editor:

print("=== String Formatting Playground ===\n")

# === % Formatting ===
name = "Alice"
age = 30
print("[%% Formatting]")
print("%s is %d years old" % (name, age))
print("Pi: %.3f" % 3.14159)
print()

# === .format() Method ===
print("[.format() Method]")
print("{} is {} years old".format(name, age))
print("{0} is {1} years old. {0} loves Python.".format(name, age))
print("Pi: {:.3f}".format(3.14159))
print()

# === F-Strings (Modern) ===
print("[F-Strings - The Best]")
name = "Bob"
age = 25
city = "London"
print(f"My name is {name} and I am {age} years old from {city}")

# Expressions inside f-strings
a, b = 15, 4
print(f"{a} + {b} = {a + b}")
print(f"{a} / {b} = {a / b:.2f}")
print(f"Big number: {123456789:,}")

# Alignment
print(f"\nAlignment demo:")
print(f"{'Left':<10} | {'Center':^10} | {'Right':>10}")
print(f"{'A':<10} | {'B':^10} | {'C':>10}")
print(f"{'10':<10} | {'20':^10} | {'30':>10}")

# Padding
print(f"\nPadding demo:")
print(f"{'Hello':*^20}")
print(f"{'Hello':=>20}")