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Module 012: Conditional Statements (if / elif / else)

Phase: 2. Control Flow & Data | Estimated time: 2 hours | Milestone Project: No

Prerequisites

Learning Objectives

  • Write if statements to execute code conditionally
  • Use elif and else to handle multiple branches
  • Properly indent code blocks
  • Nest conditionals inside each other
  • Use truthiness directly in conditions
  • Check for None, empty strings, and other common patterns

Why This Matters

Conditional statements are how programs make decisions. Without them, code runs the same way every time. With if/elif/else, your program can react to user input, validate data, and handle different scenarios — the essence of intelligent behavior.

Concept Explanation

The if Statement

An if statement runs a block of code only when its condition is truthy:

age = 18
if age >= 18:
    print("You are an adult.")
    print("You can vote.")
# Output: You are an adult.
# Output: You can vote.

The condition is any expression that Python evaluates for truthiness. The indented block (4 spaces by convention) runs only if the condition is True.

age = 16
if age >= 18:
    print("You are an adult.")  # This does NOT run
print("This runs either way.")
# Output: This runs either way.

The else Clause

Add else to run code when the condition is falsy:

age = 16
if age >= 18:
    print("You are an adult.")
else:
    print("You are a minor.")
# Output: You are a minor.

The elif Clause

Use elif (short for "else if") for multiple conditions:

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"
print(f"Grade: {grade}")
# Output: Grade: B

Only the first matching branch runs. Once a condition is True, Python skips the remaining elif and else blocks.

Indentation

Python uses indentation (4 spaces recommended) to define code blocks. Consistent indentation is mandatory:

x = 10
if x > 5:
    print("x is greater than 5")    # 4 spaces
    if x > 8:
        print("x is also > 8")      # 8 spaces (nested)
print("Outside the if block")       # 0 spaces

Truthiness in Conditions

You don't need if x == True:. Just use if x::

name = input("Enter your name: ")  # User types "" (empty)
if name:
    print(f"Hello, {name}!")
else:
    print("No name entered.")
# Output: No name entered.

Checking for None

The idiomatic way to check for None:

result = None
if result is None:
    print("No result yet.")
else:
    print(f"Result: {result}")
# Output: No result yet.

Use is None rather than == None. The is operator checks identity, which is the correct way to compare to None.

Checking for Empty Strings

text = ""
if not text:
    print("String is empty.")
# Output: String is empty.

Nested Conditionals

You can put if statements inside other if statements:

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive.")
    else:
        print("Get a license first.")
else:
    print("Too young to drive.")
# Output: You can drive.

Common Pitfalls

  1. Forgetting the colon — Every if, elif, and else line must end with a colon (:).

  2. Inconsistent indentation — Mixing tabs and spaces, or using the wrong number of spaces, causes IndentationError.

  3. Using assignment = instead of comparison == — Python 3.12 allows = in conditions, but it's almost certainly a bug if you meant ==.

  4. Putting too many conditions in elif — Remember that once a condition matches, later elif branches are skipped, even if they would also match.

  5. Empty if block — Python won't let you leave a block empty. Use pass as a placeholder:

if condition:
    pass  # I'll fill this in later

Hands-On Walkthrough

Let's build a simple number guessing game using conditionals:

import random

secret = random.randint(1, 10)
guess = int(input("Guess a number 1-10: "))  # Assume user types 7

if guess == secret:
    print("Perfect! You guessed it!")
elif guess > secret:
    print("Too high! Try again.")
else:
    print("Too low! Try again.")

print(f"The secret number was {secret}.")

Now a temperature checker with nested conditions:

temp = float(input("Enter temperature in °C: "))  # Assume 30

if temp > 30:
    print("It's hot outside!")
elif temp > 20:
    print("It's warm and pleasant.")
elif temp > 10:
    print("It's a bit cool.")
elif temp > 0:
    print("It's cold!")
else:
    print("It's freezing! Stay inside.")

Key Takeaways

  • if runs a block when its condition is truthy; else provides the fallback
  • elif adds additional conditions — only the first matching branch executes
  • Indentation (4 spaces) defines blocks; all lines in a block must be indented consistently
  • Use if x: for truthiness checks instead of if x == True:
  • Use if x is None: to check for None (identity, not equality)
  • Use if not x: to check for empty strings, empty collections, or zero values
  • Nested conditionals let you check multiple layers of conditions
  • Every if, elif, else line must end with a colon

Further Reading

Next Module

Module 013: Comparison Chaining & Logical Operators Deep Dive — Master chained comparisons like 1 < x < 10, understand is vs ==, and learn De Morgan's laws.