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String Formatting

String Formatting

Section titled “String Formatting”

Introduction

Section titled “Introduction”

Python offers three ways to format strings. f-strings (Python 3.6+) are the preferred approach.

f-strings (Preferred ✅)

Section titled “f-strings (Preferred ✅)”
name = "Alice"
age = 30
score = 98.765


# Basic
print(f"Name: {name}, Age: {age}")


# Expressions
print(f"In 10 years: {age + 10}")
print(f"Upper: {name.upper()}")


# Format specs
print(f"Score: {score:.2f}")       # 98.77
print(f"Score: {score:10.2f}")     # Right-aligned
print(f"Millions: {1_000_000:,}")  # 1,000,000


# Debugging (Python 3.8+)
x = 42
print(f"{x=}")     # x=42

.format() Method (Python 3.0+)

Section titled “.format() Method (Python 3.0+)”
name = "Alice"
age = 30


print("Name: {}, Age: {}".format(name, age))
print("{0} is {1} years old".format(name, age))
print("{name} is {age} years old".format(name="Bob", age=25))


print("{:b}".format(255))          # 11111111 (binary)
print("{:x}".format(255))          # ff (hex)
print("{:e}".format(0.001234))     # 1.234000e-03

% Formatting (Old Style)

Section titled “% Formatting (Old Style)”
name = "Alice"
age = 30
score = 98.765


print("Name: %s, Age: %d, Score: %.2f" % (name, age, score))

Format Specification Mini-Language

Section titled “Format Specification Mini-Language”
[[fill]align][sign][width][.precision][type]
print(f"{'hi':*^10}")    # '****hi****'
print(f"{42:+}")         # '+42'
print(f"{3.14159:.3f}")  # '3.142'
print(f"{255:b}")        # '11111111'
print(f"{1000000:,}")   # '1,000,000'

Why It Matters

Section titled “Why It Matters”

Proper string formatting is essential for creating readable logs, user-facing messages, and data reports.

Interview Questions

Section titled “Interview Questions”

Q1: Why are f-strings preferred over .format() and % formatting?

A: f-strings are more readable (inline expressions), faster (compile-time evaluation), and support any Python expression inside {}.

Q2: How do you round a float to 2 decimal places in an f-string?

A: f"{value:.2f}" — the .2f format specifier rounds to 2 decimal places.

Practice Exercises

Section titled “Practice Exercises”
  1. Format a number as currency with commas and 2 decimal places.
  2. Create a formatted table of student names and scores.
  3. Use f-string debugging to print variable names and values.