Conditional Statements in Python

Introduction

Conditional statements allow executing different code blocks based on certain conditions. Python uses if, elif, and else for flow control.

The if Statement

Executes a code block only if a condition is True:

age = 18

if age >= 18:
    print("You are an adult")

Indentation

Important: Python uses indentation (spaces) to delimit code blocks. Standard is 4 spaces.

if condition:
    # This block executes if condition is True
    statement1
    statement2
# Here the if block ends

The if-else Statement

Executes one block if condition is True, another if False:

age = 16

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

The if-elif-else Statement

For checking multiple conditions:

grade = 8

if grade >= 9:
    print("Excellent")
elif grade >= 7:
    print("Good")
elif grade >= 5:
    print("Satisfactory")
else:
    print("Unsatisfactory")

Order matters!

Conditions are evaluated top to bottom. The first true condition is executed:

x = 15

# Wrong - first condition is always True for x > 10
if x > 5:
    print("Greater than 5")    # Always executes
elif x > 10:
    print("Greater than 10")   # Never reached

# Correct - order from specific to general
if x > 10:
    print("Greater than 10")   # Executes
elif x > 5:
    print("Greater than 5")

Complex Conditions

Use logical operators for multiple conditions:

age = 25
has_license = True

# AND - both conditions must be True
if age >= 18 and has_license:
    print("Can drive")

# OR - at least one condition True
grade = 4
if grade >= 5 or grade == 4:
    print("Accepted")

# NOT - negates the condition
is_raining = False
if not is_raining:
    print("We can go outside")

Nested If Statements

An if can be inside another if:

age = 20
has_license = True

if age >= 18:
    if has_license:
        print("Can drive")
    else:
        print("Needs to get license")
else:
    print("Too young for license")

Conditional Expression (Ternary Operator)

A compact form for conditional assignments:

# Syntax: value_true if condition else value_false

age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult

# Equivalent to:
if age >= 18:
    status = "adult"
else:
    status = "minor"

Practical examples:

# Absolute value
x = -5
abs_x = x if x >= 0 else -x  # 5

# Minimum of two numbers
a, b = 10, 5
minimum = a if a < b else b  # 5

# Parity
n = 7
parity = "even" if n % 2 == 0 else "odd"  # odd

The pass Statement

pass is a placeholder for empty blocks:

x = 10

if x > 5:
    pass  # TODO: implement later
else:
    print("x is small")

“Truthy” and “Falsy” Values

In conditions, the following evaluate as False:

• False
• None
• 0, 0.0
• "" (empty string)
• [], (), {} (empty collections)

All other values are True:

lst = [1, 2, 3]

# Check if list is not empty
if lst:
    print("List has elements")

# Better than:
if len(lst) > 0:
    print("List has elements")

Comparing with None

Use is for comparing with None:

result = None

# Correct
if result is None:
    print("No result")

# Works but not recommended
if result == None:
    print("No result")

Common Mistakes

1. Forgetting the colon

if x > 5     # SyntaxError: expected ':'
    print("Big")

if x > 5:    # Correct
    print("Big")

2. Incorrect indentation

if x > 5:
print("Big")  # IndentationError

if x > 5:
    print("Big")  # Correct

3. Using = instead of ==

if x = 5:    # SyntaxError
    print("Five")

if x == 5:   # Correct
    print("Five")

4. Non-mutually exclusive conditions

# Both can execute
if x > 5:
    print("Big")
if x > 3:
    print("This too")  # Also executes!

# Only one executes
if x > 5:
    print("Big")
elif x > 3:
    print("Medium")

Key Points for Exam

• Indentation defines blocks (4 spaces standard)
• Conditions are evaluated top to bottom
• First true condition stops evaluation
• Ternary operator: val_true if cond else val_false
• “Falsy” values: False, None, 0, "", [], {}
• Use is None, not == None

Review Questions

1. What does print("Yes" if 0 else "No") display?
2. How many elif clauses can you have in an if?
3. What happens if you forget : after if?
4. What does it mean that an empty list is “falsy”?