Python if, else, and elif Conditional Statements: A Complete Guide

Conditional statements are fundamental to programming logic in Python. They allow your programs to make decisions, execute different code paths, and respond intelligently to varying inputs and conditions. Whether you're building a simple calculator or a complex data analysis tool, mastering if, else, and elif statements is essential.

This comprehensive guide will teach you everything you need to know about Python's conditional statements, from basic syntax to advanced patterns and best practices.

What are Conditional Statements?

Conditional statements are code structures that execute specific blocks of code only when certain conditions are met. They enable your programs to:

• Make decisions based on user input
• Validate data before processing
• Handle different scenarios appropriately
• Control program flow dynamically
• Respond to changing conditions

Python provides three keywords for conditional logic:

• if - Execute code when a condition is True
• elif - Check additional conditions (short for "else if")
• else - Execute code when all conditions are False

The if Statement

The simplest conditional statement is if. It executes a block of code only when a specified condition evaluates to True.

Basic Syntax

if condition:
    # Code to execute if condition is True
    statement1
    statement2

Important: Python uses indentation (typically 4 spaces) to define code blocks. All indented statements after the if are part of the conditional block.

Simple Example

age = 20

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

# Output:
# You are an adult.
# You can vote.

If the condition is False, the indented block is skipped:

age = 15

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

print("This always runs.")

# Output:
# This always runs.

Comparison Operators

Conditions typically use comparison operators to evaluate expressions:

Operator	Meaning	Example
==	Equal to	x == 5
!=	Not equal to	x != 5
>	Greater than	x > 5
<	Less than	x < 5
>=	Greater than or equal to	x >= 5
<=	Less than or equal to	x <= 5

Examples

temperature = 75

if temperature > 80:
    print("It's hot outside!")

if temperature <= 80:
    print("The temperature is comfortable.")

# Output: The temperature is comfortable.

The else Statement

The else statement provides an alternative block of code that executes when the if condition is False.

Syntax

if condition:
    # Execute if condition is True
else:
    # Execute if condition is False

Example

age = 15

if age >= 18:
    print("You can vote.")
else:
    print("You are not old enough to vote yet.")

# Output: You are not old enough to vote yet.

Practical Example: Even or Odd

number = 7

if number % 2 == 0:
    print(f"{number} is even.")
else:
    print(f"{number} is odd.")

# Output: 7 is odd.

The elif Statement (else-if)

When you need to check multiple conditions sequentially, use elif (short for "else if"). Python evaluates conditions from top to bottom and executes the first True block, then skips the rest.

Syntax

if condition1:
    # Execute if condition1 is True
elif condition2:
    # Execute if condition1 is False and condition2 is True
elif condition3:
    # Execute if condition1 and condition2 are False and condition3 is True
else:
    # Execute if all conditions are False

Example: Grade Calculator

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"Your grade is: {grade}")
# Output: Your grade is: B

How elif Works

Once a condition is True, Python executes that block and skips all remaining elif and else blocks, even if their conditions would also be True.

number = 15

if number > 10:
    print("Number is greater than 10")
elif number > 5:
    print("Number is greater than 5")  # This won't execute
else:
    print("Number is 5 or less")       # This won't execute

# Output: Number is greater than 10

Multiple Conditions with Logical Operators

You can combine multiple conditions using logical operators:

and Operator

Both conditions must be True:

age = 25
has_license = True

if age >= 18 and has_license:
    print("You can drive.")
else:
    print("You cannot drive.")

# Output: You can drive.

or Operator

At least one condition must be True:

is_weekend = False
is_holiday = True

if is_weekend or is_holiday:
    print("No work today!")
else:
    print("Time to work.")

# Output: No work today!

not Operator

Negates a condition:

is_raining = False

if not is_raining:
    print("You don't need an umbrella.")
else:
    print("Take an umbrella.")

# Output: You don't need an umbrella.

Combining Multiple Operators

age = 25
has_ticket = True
has_id = True

if age >= 18 and (has_ticket and has_id):
    print("You can enter the venue.")
elif age >= 18 and not has_ticket:
    print("You need to buy a ticket.")
else:
    print("You are not old enough to enter.")

# Output: You can enter the venue.

Nested if Statements

You can place if statements inside other if statements to create more complex decision trees.

Syntax

if condition1:
    if condition2:
        # Execute if both condition1 and condition2 are True
    else:
        # Execute if condition1 is True but condition2 is False
else:
    # Execute if condition1 is False

Example: Authentication System

username = "admin"
password = "secure123"

if username == "admin":
    if password == "secure123":
        print("Access granted!")
    else:
        print("Incorrect password.")
else:
    print("User not found.")

# Output: Access granted!

Example: BMI Calculator

weight = 70  # kg
height = 1.75  # meters

bmi = weight / (height ** 2)

if bmi < 18.5:
    category = "Underweight"
    if bmi < 16:
        print("Severely underweight - please consult a doctor")
    else:
        print("Underweight - consider gaining weight")
elif bmi < 25:
    category = "Normal"
    print("Healthy weight - keep it up!")
elif bmi < 30:
    category = "Overweight"
    print("Overweight - consider lifestyle changes")
else:
    category = "Obese"
    if bmi >= 35:
        print("Severely obese - please consult a doctor")
    else:
        print("Obese - consider medical advice")

print(f"BMI: {bmi:.1f} - Category: {category}")

# Output:
# Healthy weight - keep it up!
# BMI: 22.9 - Category: Normal

Note: Deep nesting (more than 2-3 levels) can make code hard to read. Consider refactoring into functions or simplifying logic.

Ternary Conditional Expression (One-Line if-else)

Python supports a compact one-line syntax for simple if-else statements:

Syntax

value_if_true if condition else value_if_false

Examples

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

# Traditional equivalent:
if age >= 18:
    status = "adult"
else:
    status = "minor"

number = -5
result = "positive" if number > 0 else "negative or zero"
print(result)  # Output: negative or zero

temperature = 75
clothing = "light clothes" if temperature > 70 else "warm clothes"
print(f"Wear {clothing}")  # Output: Wear light clothes

Limitation: Ternary expressions only work for simple if-else (no elif support).

Truthy and Falsy Values

In Python, not all conditions explicitly use comparison operators. Python evaluates values as "truthy" or "falsy" in boolean contexts.

Falsy Values

These values evaluate to False:

• None
• False
• Zero of any numeric type: 0, 0.0, 0j
• Empty sequences: '', [], ()
• Empty mappings: {}
• Empty sets: set()

Truthy Values

All other values evaluate to True, including:

• Non-zero numbers
• Non-empty strings, lists, tuples, dictionaries
• True

Examples

username = ""

if username:
    print(f"Hello, {username}!")
else:
    print("Please enter a username.")

# Output: Please enter a username.

items = [1, 2, 3]

if items:
    print("The list has items.")
else:
    print("The list is empty.")

# Output: The list has items.

value = None

if value:
    print("Value exists.")
else:
    print("Value is None or empty.")

# Output: Value is None or empty.

Practical Examples

Example 1: Login System

def authenticate(username, password):
    """Simple authentication function"""
    valid_users = {
        "admin": "admin123",
        "user": "pass123"
    }

    if username not in valid_users:
        return "Error: User not found"
    elif valid_users[username] != password:
        return "Error: Incorrect password"
    else:
        return f"Welcome, {username}!"

# Test the function
print(authenticate("admin", "admin123"))  # Output: Welcome, admin!
print(authenticate("user", "wrong"))      # Output: Error: Incorrect password
print(authenticate("guest", "123"))       # Output: Error: User not found

Example 2: Discount Calculator

def calculate_discount(total, customer_type):
    """Calculate discount based on customer type and total"""
    if total < 0:
        return "Error: Invalid total"

    if customer_type == "VIP":
        if total >= 1000:
            discount = 0.30
        elif total >= 500:
            discount = 0.20
        else:
            discount = 0.15
    elif customer_type == "Regular":
        if total >= 500:
            discount = 0.10
        else:
            discount = 0.05
    else:
        discount = 0.0  # No discount for guests

    final_total = total * (1 - discount)
    return f"Original: ${total:.2f}, Discount: {discount*100:.0f}%, Final: ${final_total:.2f}"

print(calculate_discount(1200, "VIP"))
# Output: Original: $1200.00, Discount: 30%, Final: $840.00

print(calculate_discount(300, "Regular"))
# Output: Original: $300.00, Discount: 5%, Final: $285.00

Example 3: Temperature Converter with Validation

def convert_temperature(value, from_unit, to_unit):
    """Convert temperature between Celsius, Fahrenheit, and Kelvin"""
    # Validate input
    if from_unit not in ["C", "F", "K"] or to_unit not in ["C", "F", "K"]:
        return "Error: Invalid unit. Use C, F, or K."

    # Check absolute zero
    if from_unit == "C" and value < -273.15:
        return "Error: Temperature below absolute zero"
    elif from_unit == "F" and value < -459.67:
        return "Error: Temperature below absolute zero"
    elif from_unit == "K" and value < 0:
        return "Error: Temperature below absolute zero"

    # Convert to Celsius first
    if from_unit == "F":
        celsius = (value - 32) * 5/9
    elif from_unit == "K":
        celsius = value - 273.15
    else:
        celsius = value

    # Convert from Celsius to target unit
    if to_unit == "F":
        result = celsius * 9/5 + 32
    elif to_unit == "K":
        result = celsius + 273.15
    else:
        result = celsius

    return f"{value}°{from_unit} = {result:.2f}°{to_unit}"

print(convert_temperature(100, "C", "F"))  # Output: 100°C = 212.00°F
print(convert_temperature(32, "F", "C"))   # Output: 32°F = 0.00°C
print(convert_temperature(-300, "C", "K")) # Output: Error: Temperature below absolute zero

Example 4: Traffic Light System

def traffic_light(color, pedestrian_waiting=False):
    """Determine action based on traffic light color"""
    color = color.lower()

    if color == "green":
        if pedestrian_waiting:
            return "Proceed with caution - pedestrian may cross"
        else:
            return "Go - traffic is clear"
    elif color == "yellow":
        return "Slow down - light will turn red soon"
    elif color == "red":
        return "Stop - wait for green light"
    else:
        return "Error: Invalid traffic light color"

print(traffic_light("green"))
# Output: Go - traffic is clear

print(traffic_light("green", pedestrian_waiting=True))
# Output: Proceed with caution - pedestrian may cross

print(traffic_light("red"))
# Output: Stop - wait for green light

Common Mistakes and How to Avoid Them

1. Using = Instead of ==

# Wrong - This assigns 5 to x, doesn't compare
if x = 5:  # SyntaxError
    print("x is 5")

# Correct - This compares x to 5
if x == 5:
    print("x is 5")

2. Forgetting Indentation

# Wrong - IndentationError
if age >= 18:
print("You can vote")

# Correct
if age >= 18:
    print("You can vote")

3. elif Without if

# Wrong - SyntaxError
elif x > 5:
    print("Greater than 5")

# Correct
if x > 10:
    print("Greater than 10")
elif x > 5:
    print("Greater than 5")

4. Confusing if with elif

x = 15

# Multiple if statements - all conditions are checked
if x > 10:
    print("Greater than 10")    # Executes
if x > 5:
    print("Greater than 5")     # Also executes

# if-elif chain - only first True condition executes
if x > 10:
    print("Greater than 10")    # Executes
elif x > 5:
    print("Greater than 5")     # Skipped

5. Not Using Parentheses for Complex Conditions

# Confusing
if age >= 18 and has_license or has_permit:
    # Is it (age >= 18 and has_license) or has_permit?
    # Or age >= 18 and (has_license or has_permit)?
    pass

# Clear
if (age >= 18 and has_license) or has_permit:
    pass

Best Practices

1. Keep Conditions Simple and Readable

# Bad - Hard to read
if not (age < 18 or not has_license):
    drive()

# Good - Clear intent
if age >= 18 and has_license:
    drive()

2. Use Meaningful Variable Names

# Bad
if x and y:
    z = True

# Good
if user_logged_in and has_permission:
    allow_access = True

3. Avoid Deep Nesting

# Bad - Deep nesting
if condition1:
    if condition2:
        if condition3:
            if condition4:
                do_something()

# Good - Early returns or combined conditions
if not condition1:
    return

if not condition2:
    return

if not condition3:
    return

if condition4:
    do_something()

# Or combine conditions
if condition1 and condition2 and condition3 and condition4:
    do_something()

4. Use elif Instead of Multiple ifs for Mutually Exclusive Conditions

# Less efficient - checks all conditions
if score >= 90:
    grade = "A"
if score >= 80 and score < 90:
    grade = "B"
if score >= 70 and score < 80:
    grade = "C"

# Better - stops after first match
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"

5. Consider Using Dictionary Mapping for Many Conditions

# Instead of many elif statements
def get_day_name(day_number):
    if day_number == 1:
        return "Monday"
    elif day_number == 2:
        return "Tuesday"
    elif day_number == 3:
        return "Wednesday"
    # ... many more

# Better - Dictionary mapping
def get_day_name(day_number):
    days = {
        1: "Monday",
        2: "Tuesday",
        3: "Wednesday",
        4: "Thursday",
        5: "Friday",
        6: "Saturday",
        7: "Sunday"
    }
    return days.get(day_number, "Invalid day")

Conclusion

Conditional statements (if, elif, else) are fundamental to programming logic in Python. They enable your programs to make intelligent decisions, validate input, handle different scenarios, and control program flow based on dynamic conditions.

Key takeaways:

1. if - Executes code when a condition is True
2. elif - Checks additional conditions sequentially (only if previous conditions were False)
3. else - Provides a fallback when all conditions are False
4. Use comparison operators (==, !=, >, <, >=, <=) to create conditions
5. Combine conditions with logical operators (and, or, not)
6. Nested if statements allow complex decision trees
7. Ternary expressions provide compact one-line if-else syntax
8. Follow best practices: keep conditions simple, use meaningful names, avoid deep nesting

Master these conditional statements, and you'll be able to write Python programs that respond intelligently to any situation. Practice with real-world examples, start simple, and gradually build more complex logic as you become comfortable with the basics.

Happy coding!