Python Keywords Complete Guide

# if, elif, else - Complete example
score = 85

if score >= 90:
    grade = "A"
    print("Excellent!")
elif score >= 80:
    grade = "B"
    print("Good job!")
elif score >= 70:
    grade = "C"
    print("You passed!")
elif score >= 60:
    grade = "D"
    print("You passed, but need improvement.")
else:
    grade = "F"
    print("Sorry, you failed.")

print(f"Score: {score}, Grade: {grade}")

# Nested if statements
age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive!")
    else:
        print("You need a license to drive.")
else:
    print("You're too young to drive.")

# for loop with break and continue
print("=== for loop examples ===")
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# Using break to exit loop early
for num in numbers:
    if num == 5:
        print("Found 5, breaking loop!")
        break
    print(num)

print("\n=== Using continue ===")
# Using continue to skip specific items
for num in numbers:
    if num % 2 == 0:  # Skip even numbers
        continue
    print(f"Odd number: {num}")

print("\n=== while loop example ===")
# while loop with condition
count = 0
while count < 5:
    print(f"Count: {count}")
    count += 1

# Infinite loop with break
print("\n=== Infinite loop with break ===")
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input.lower() == 'quit':
        print("Goodbye!")
        break
    print(f"You entered: {user_input}")

# Function definition with def and return
def calculate_area(radius):
    """Calculate area of circle"""
    pi = 3.14159
    area = pi * radius ** 2
    return area

# Lambda function (anonymous function)
square = lambda x: x ** 2

# Class definition
class Rectangle:
    """Rectangle class with area calculation"""
    
    def __init__(self, width, height):
        # Constructor method
        self.width = width
        self.height = height
    
    def area(self):
        """Calculate area of rectangle"""
        return self.width * self.height
    
    def perimeter(self):
        """Calculate perimeter"""
        return 2 * (self.width + self.height)

# Using the function and class
circle_area = calculate_area(5)
print(f"Area of circle with radius 5: {circle_area:.2f}")

print(f"Square of 4 using lambda: {square(4)}")

# Create rectangle object
rect = Rectangle(10, 5)
print(f"Rectangle area: {rect.area()}")
print(f"Rectangle perimeter: {rect.perimeter()}")

# Basic try-except
try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print(f"100 divided by {number} is {result}")
except ValueError:
    print("Error: That's not a valid number!")
except ZeroDivisionError:
    print("Error: Cannot divide by zero!")
except Exception as e:
    print(f"An unexpected error occurred: {e}")
finally:
    print("This always executes, with or without errors.")

# Using raise to create custom exceptions
def validate_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative!")
    elif age > 150:
        raise ValueError("Age seems unrealistic!")
    return True

# Using assert for debugging
def calculate_discount(price, discount_rate):
    assert 0 <= discount_rate <= 1, "Discount rate must be between 0 and 1"
    return price * (1 - discount_rate)

# Test the functions
try:
    validate_age(-5)
except ValueError as e:
    print(f"Validation error: {e}")

print(f"Discount price: {calculate_discount(100, 0.2)}")