Introduction to Conditionals

Conditional statements allow your program to execute certain pieces of code based on specific conditions. They enable decision-making capabilities in your code.

Basic if Statement

Indentation Notice

Take note of the indentation after :

age = 20
if age >= 18:
    print("You are eligible to vote.")

Output:

You are eligible to vote.

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if...else Statement

The if...else statement executes one block of code if a condition is true and another block if it is false.

temperature = 15
if temperature > 20:
    print("It's warm outside.")
else:
    print("It's cold outside.")

Output:

It's cold outside.

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if...elif...else Statement

For multiple conditions, use elif (short for "else if").

score = 85
if score >= 90:
    grade = 'A'
elif score >= 80:
    grade = 'B'
elif score >= 70:
    grade = 'C'
else:
    grade = 'D'
print(f"Your grade is {grade}.")

Output:

Your grade is B.

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Nested Conditionals

Conditionals can be nested within each other for more complex logic.

username = "admin"
password = "1234"

if username == "admin":
    if password == "1234":
        print("Access granted.")
    else:
        print("Incorrect password.")
else:
    print("Username not recognized.")

Output:

Access granted.

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Comparison Operators

Comparison operators compare two values:

• == Equal to
• != Not equal to
• > Greater than
• < Less than
• >= Greater than or equal to
• <= Less than or equal to

Example

budget = 500
price = 450

if price <= budget:
    print("You can purchase this item.")
else:
    print("This item is too expensive.")

Output:

You can purchase this item.

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Logical Operators

Logical operators combine multiple conditions:

• and True if both conditions are true
• or True if at least one condition is true
• not Inverts the truth value

Using and

age = 25
has_license = True

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

Output:

You can drive.

Using or

has_ticket = False
is_on_guest_list = True

if has_ticket or is_on_guest_list:
    print("You may enter the concert.")
else:
    print("You cannot enter the concert.")

Output:

You may enter the concert.

Using not

is_raining = False

if not is_raining:
    print("No need for an umbrella.")

Output:

No need for an umbrella.

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Membership Operators

Membership operators test for membership in a sequence:

• in True if value is in sequence
• not in True if value is not in sequence

Example with in

available_colors = ['red', 'green', 'blue']
if 'green' in available_colors:
    print("Green color is available.")

Output:

Green color is available.

Example with not in

blocked_users = ['user123', 'badguy']
current_user = 'john_doe'

if current_user not in blocked_users:
    print("Access granted.")
else:
    print("Access denied.")

Output:

Access granted.

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Identity Operators

Identity operators compare the memory locations of two objects:

• is True if both variables point to the same object
• is not True if variables point to different objects

Example

a = [1, 2, 3]
b = a
c = [1, 2, 3]

print(a is b)   # True
print(a is c)   # False

Output:

True
False

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Ternary Conditional Operator

A one-line if...else statement.

status = "Success" if operation_completed else "Failed"
print(status)

Real-World Example

age = 17
message = "Eligible to vote" if age >= 18 else "Not eligible to vote"
print(message)

Output:

Not eligible to vote

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Combining Conditions

You can combine multiple conditions using logical operators.

day = "Saturday"
time = 14

if day in ["Saturday", "Sunday"] and 9 <= time <= 17:
    print("The store is open.")
else:
    print("The store is closed.")

Output:

The store is open.

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Short-Circuit Evaluation

Python stops evaluating conditions as soon as the result is determined.

def check_user(user):
    print("Checking user...")
    return user == "admin"

def check_password(pwd):
    print("Checking password...")
    return pwd == "secret"

if check_user("admin") and check_password("secret"):
    print("Access granted.")

Output:

Checking user...
Checking password...
Access granted.

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Real-World Examples

Email Validation

email = "[email protected]"

if "@" in email and "." in email:
    print("Valid email address.")
else:
    print("Invalid email address.")

Output:

Valid email address.

Shopping Cart Discount

cart_total = 120
is_member = True

if cart_total > 100 or is_member:
    print("You qualify for free shipping.")
else:
    print("Shipping charges apply.")

Output:

You qualify for free shipping.

Password Strength Checker

password = "Password123!"

if len(password) >= 8:
    if any(char.isdigit() for char in password):
        if any(char.isupper() for char in password):
            if any(not char.isalnum() for char in password):
                print("Strong password.")
            else:
                print("Password should include a special character.")
        else:
            print("Password should include an uppercase letter.")
    else:
        print("Password should include a number.")
else:
    print("Password should be at least 8 characters long.")

Output:

Strong password.

Traffic Light System

light_color = "Yellow"

if light_color == "Green":
    action = "Go"
elif light_color == "Yellow":
    action = "Slow down"
elif light_color == "Red":
    action = "Stop"
else:
    action = "Proceed with caution"

print(f"The light is {light_color}. You should {action}.")

Output:

The light is Yellow. You should Slow down.

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The pass Statement

Used when a statement is syntactically required but you don't want any command to execute.

if False:
    pass  # Placeholder for future code
else:
    print("This will execute.")

Output:

This will execute.

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The match Statement (Python 3.10+)

Pattern matching allows for cleaner syntax when matching against multiple possibilities.

command = "pause"

match command:
    case "start":
        print("Starting the system.")
    case "stop":
        print("Stopping the system.")
    case "pause":
        print("Pausing the system.")
    case _:
        print("Unknown command.")

Output:

Pausing the system.

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