Control Flow

if, elif, and else statements

These statements let your program make decisions based on conditions — they’re the core of control flow in Python. For beginners, see our Introduction to Python lesson to get started, and our Loops and Iteration module for how these conditions interact with repeatable tasks.

Basic Syntax

if condition:
# code block if condition is True
elif another_condition:
# code block if elif is True
else:
# code block if none are True

• Each condition is checked in order
• Indentation (typically 4 spaces) defines code blocks
• You can have one if, zero or more elif, and zero or one else

Simple Example:

temperature = 25

if temperature > 30:
print(“It’s hot outside.”)
elif temperature >= 20:
print(“It’s a nice day.”)
else:
print(“It’s cold outside.”)

Output: “It’s a nice day.”

Flow Explanation:

• If the if condition is True, it runs that block and skips the rest.
• If the if is False, Python checks the elif (if any).
• If none of the conditions are True, the else block runs.

Real-Life Example: Age Check

age = int(input(“Enter your age: “))

if age < 13:
print(“You are a child.”)
elif age < 18:
print(“You are a teenager.”)
else:
print(“You are an adult.”)

Nestings and Practical Use

You can nest conditions inside others:

score = 85

if score >= 50:
print(“You passed!”)
if score >= 80:
print(“Great job!”)
else:
print(“You failed.”)

Common Mistakes to Avoid

Mistake	Fix
Missing colon :	Add a colon at end of condition if x == 5 → if x == 5:
Improper indentation	Use consistent 4-space indentation print("Hello") → print("Hello")
Using = instead of ==	= is assignment, == is comparison if x = 5: → if x == 5:
Forgetting to cast input() to int	Use int(input()) if comparing numbers age = input() → age = int(input())

Quick Quiz

What will this print?

x = 10

if x > 20:
print(“A”)
elif x > 5:
print(“B”)
else:
print(“C”)

Answer: “B”

Summary Table:

Statement	Purpose
if	Starts a conditional check
elif	Checks another condition if the previous if or elif fails
else	Runs if no if or elif conditions are met

Truthy and falsy values

In Python, every value has an inherent Boolean value — either True or False — even if it’s not literally the TrueorFalse` Boolean type. For a deeper understanding of how Python determines truthiness, check our Variables, Data Types and Operators lesson.

These values are evaluated when used in conditions, such as in if statements. For more on how different types behave in Boolean contexts, our Introduction to Python overview is a great starting point.

What are “Truthy” and “Falsy”?

• Truthy: A value that evaluates to True when used in a Boolean context.
• Falsy: A value that evaluates to False.

Example:

if “hello”:
print(“This is truthy.”)

Output: This is truthy.

• Because a non-empty string (“hello”) is considered truthy.

Falsy Values in Python

These values are considered False in Boolean contexts:

Type	Falsy Value
Boolean	False
NoneType	None
Numeric	0, 0.0, 0j
Sequence/Collection	'' (empty string), [] (empty list), () (empty tuple), {} (empty dict), set() (empty set)
Custom object	Any object with __bool__() or __len__() returning False

# All of these evaluate as False
if not “”:
print(“Empty string is falsy”)

if not 0:
print(“Zero is falsy”)

if not None:
print(“None is falsy”)

if not []:
print(“Empty list is falsy”)

Truthy Values in Python

Everything else not listed as falsy is truthy.

Examples:

“hello” # non-empty string
42 # any non-zero number
[1, 2, 3] # non-empty list
{“a”: 1} # non-empty dictionary
True # obviously!

These will all make if statements run:

if [1, 2, 3]:
print(“Truthy list!”)

if “Python”:
print(“Truthy string!”)

Boolean Context Examples:

x = []

if x:
print(“List has items.”)
else:
print(“List is empty.”)

Output: List is empty.

Using bool() to Check Value Truthiness

You can test how Python evaluates any value using bool():

print(bool(0)) # False
print(bool(“Hi”)) # True
print(bool([])) # False
print(bool([0])) # True (non-empty list!)

Summary:

Value Type	Truthy Example	Falsy Example
String	"Python"	""
Number	42	0
List	[1, 2]	[]
Dictionary	{"a": 1}	{}
Boolean	True	False
NoneType	—	None

Nested conditionals

What Are Nested Conditionals?

• A nested conditional is when you place an if, elif, or else inside another if block.
• This lets you check multiple layers of logic, like decision trees.

Basic Syntax:

if condition1:
if condition2:
# Runs if both condition1 AND condition2 are True
else:
# Runs if condition1 is True but condition2 is False
else:
# Runs if condition1 is False

Simple Example:

age = 20
has_ticket = True

if age >= 18:
if has_ticket:
print(“You can enter the event.”)
else:
print(“You need a ticket to enter.”)
else:
print(“You must be 18 or older to enter.”)

Output:

You can enter the event.

Why Use Nested Conditionals?

• When one condition depends on another
• To create multi-level decision structures
• When you want to check a second condition only if the first is True

Example: Student Grading System

grade = 85

if grade >= 50:
if grade >= 90:
print(“A+”)
elif grade >= 80:
print(“A”)
else:
print(“Pass”)
else:
print(“Fail”)

Output:

A

Alternative: Combine Conditions with and

You can sometimes flatten nested conditionals by using and:

if age >= 18 and has_ticket:
print(“You can enter.”)
elif age >= 18:
print(“Get a ticket.”)
else:
print(“You’re too young.”)

Both versions are valid — use nesting for clarity when conditions are hierarchical.

Common Mistakes to Avoid

Mistake	Fix