If Statements

Beginner ~20 min read

Conditional statements let your program make decisions! The if statement is the cornerstone of control flow - it executes code only when a specific condition is true, allowing your programs to respond dynamically to different situations.

The Basic `if` Statement

The if statement executes a block of code only when the condition evaluates to True. Python uses indentation to define the code block.

Output

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Indentation Matters! Python uses indentation (typically 4 spaces) to define code blocks. All lines with the same indentation after an if statement belong to that block. This is different from languages that use braces {}.

The `if...else` Statement

Add an else clause to handle the case when the condition is False. Either the if block OR the else block will execute - never both!

Output

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Pro Tip: You can check if a list is empty by using it directly as the condition: if my_list: is True when the list has items, False when empty. This works for strings, dicts, and other collections too!

The `if...elif...else` Chain

Use elif (short for "else if") when you have multiple conditions to check. Python evaluates conditions from top to bottom and executes only the FIRST matching block.

# If-Elif-Else Chain

# Use elif when you have multiple conditions to check
# Only ONE block will execute

score = 78

print(f"Score: {score}")
print()

if score >= 90:
    grade = "A"
    message = "Excellent!"
elif score >= 80:
    grade = "B"
    message = "Good job!"
elif score >= 70:
    grade = "C"
    message = "Satisfactory"
elif score >= 60:
    grade = "D"
    message = "Needs improvement"
else:
    grade = "F"
    message = "Please see the teacher"

print(f"Grade: {grade}")
print(f"Message: {message}")

print()

# Day of the week example
day = 3  # 1 = Monday, 7 = Sunday

if day == 1:
    day_name = "Monday"
elif day == 2:
    day_name = "Tuesday"
elif day == 3:
    day_name = "Wednesday"
elif day == 4:
    day_name = "Thursday"
elif day == 5:
    day_name = "Friday"
elif day == 6:
    day_name = "Saturday"
elif day == 7:
    day_name = "Sunday"
else:
    day_name = "Invalid day"

print(f"Day {day} is {day_name}")

print()

# Age category example
age = 25

if age < 0:
    category = "Invalid age"
elif age < 13:
    category = "Child"
elif age < 20:
    category = "Teenager"
elif age < 60:
    category = "Adult"
else:
    category = "Senior"

print(f"Age {age}: {category}")

Output

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Order Matters! In an if-elif chain, conditions are checked top to bottom. Once a condition is True, the rest are skipped. Put more specific conditions before general ones: check score >= 90 before score >= 80.

Nested If Statements

You can place if statements inside other if statements for more complex decision trees. Each level adds another 4 spaces of indentation.

# Nested If Statements

# Checking multiple related conditions
is_weekend = True
weather = "sunny"
has_money = True

print(f"Weekend: {is_weekend}")
print(f"Weather: {weather}")
print(f"Has money: {has_money}")
print()

if is_weekend:
    print("It's the weekend!")

if weather == "sunny":
        print("  The weather is nice!")

if has_money:
            print("    Let's go to an amusement park!")
        else:
            print("    Let's have a picnic in the park!")
    elif weather == "rainy":
        print("  It's raining...")
        print("    Let's stay home and watch movies.")
    else:
        print("  Weather is okay.")
        print("    Let's visit a museum.")
else:
    print("It's a weekday.")
    print("  Time to work!")

print()

# Login system example
print("=== Login System ===")
username = "admin"
password = "secret123"
is_active = True

if username == "admin":
    print("Admin account found.")

if password == "secret123":
        print("  Password correct!")

if is_active:
            print("    Login successful! Welcome, Admin.")
        else:
            print("    Account is deactivated. Contact support.")
    else:
        print("  Wrong password!")
else:
    print("User not found.")

Output

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Readability Tip: Deeply nested if statements (more than 3 levels) are hard to read. Consider using early returns, breaking into functions, or using and/or to combine conditions.

Truthy and Falsy Values

Python treats certain values as "falsy" (equivalent to False in a condition) and everything else as "truthy". This allows for cleaner condition checks!

# Truthy and Falsy Values in Conditions

# Python evaluates these as False (falsy):
# - False
# - None
# - 0 (zero)
# - 0.0 (zero float)
# - "" (empty string)
# - [] (empty list)
# - {} (empty dict)
# - () (empty tuple)
# - set() (empty set)

# Everything else is True (truthy)

print("=== Falsy Values ===")
falsy_values = [False, None, 0, 0.0, "", [], {}, (), set()]

for value in falsy_values:
    if value:
        print(f"{repr(value):12} is truthy")
    else:
        print(f"{repr(value):12} is falsy")

print()

print("=== Truthy Values ===")
truthy_values = [True, 1, -1, "hello", [1, 2], {"a": 1}, (1,), {1}]

for value in truthy_values:
    if value:
        print(f"{repr(value):12} is truthy")
    else:
        print(f"{repr(value):12} is falsy")

print()

# Practical example: Checking if list has items
print("=== Practical Examples ===")
items = ["apple", "banana"]

# Instead of:
if len(items) > 0:
    print("List has items (using len)")

# You can simply write:
if items:
    print("List has items (using truthy check)")

print()

# Checking for None or empty string
name = ""

if not name:
    print("Name is empty or None - please provide a name")
else:
    print(f"Hello, {name}!")

Output

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Common Mistakes

1. Using = instead of == in conditions

# Wrong - this is assignment, not comparison!
if x = 5:  # SyntaxError!
    print("x is 5")

# Correct - use == for comparison
if x == 5:
    print("x is 5")

2. Forgetting the colon

# Wrong - missing colon!
if age >= 18
    print("Adult")  # SyntaxError!

# Correct
if age >= 18:
    print("Adult")

3. Inconsistent indentation

# Wrong - mixed indentation!
if condition:
    print("Line 1")
  print("Line 2")  # IndentationError!

# Correct - consistent 4 spaces
if condition:
    print("Line 1")
    print("Line 2")

4. Comparing with True/False explicitly

# Not recommended
if is_valid == True:
    print("Valid")

# Better - cleaner and more Pythonic
if is_valid:
    print("Valid")

# For checking False, use 'not'
if not is_valid:
    print("Not valid")

Exercise: Grade Calculator

Task: Create a grading system that assigns letter grades based on scores.

Requirements:

90-100: Grade "A"
80-89: Grade "B"
70-79: Grade "C"
60-69: Grade "D"
Below 60: Grade "F"

Output

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Show Solution

score = 75
grade = ""

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"Score: {score}, Grade: {grade}")

Summary

if: Executes code block when condition is True
else: Executes when no preceding condition was True
elif: Checks additional conditions (can have multiple)
Indentation: Python uses 4 spaces to define code blocks
Colon: Every if/elif/else line must end with :
Falsy values: False, None, 0, "", [], {}, (), set()
Truthy: Everything else is considered True in conditions

What's Next?

Now that you can make decisions with if statements, let's learn about the ternary operator - a concise one-line way to write simple if-else expressions that's perfect for assigning values based on conditions!

Previous Membership Operators

Next Ternary Operator

The Basic if Statement

The if...else Statement

The if...elif...else Chain

Nested If Statements

Truthy and Falsy Values

Common Mistakes

1. Using = instead of == in conditions

2. Forgetting the colon

3. Inconsistent indentation

4. Comparing with True/False explicitly

Exercise: Grade Calculator

Summary

What's Next?

Enjoying these tutorials?

The Basic `if` Statement

The `if...else` Statement

The `if...elif...else` Chain