#!/usr/bin/env python3
"""
Simple calculator with basic arithmetic operations.
Handles division by zero and provides simple testing.
"""

def add(a: float, b: float) -> float:
    """Return the sum of a and b."""
    return a + b

def subtract(a: float, b: float) -> float:
    """Return a minus b."""
    return a - b

def multiply(a: float, b: float) -> float:
    """Return a multiplied by b."""
    return a * b

def divide(a: float, b: float) -> float:
    """
    Return a divided by b.

    Raises:
        ZeroDivisionError: if b is 0
    """
    if b == 0:
        raise ZeroDivisionError("Cannot divide by zero")
    return a / b

def test_calculator():
    """Test all calculator functions."""
    print("Testing calculator functions...")

    # Test addition
    result = add(5, 3)
    expected = 8
    assert result == expected, f"Addition failed: {result} != {expected}"
    print(f"[OK] add(5, 3) = {result}")

    # Test subtraction
    result = subtract(10, 4)
    expected = 6
    assert result == expected, f"Subtraction failed: {result} != {expected}"
    print(f"[OK] subtract(10, 4) = {result}")

    # Test multiplication
    result = multiply(7, 6)
    expected = 42
    assert result == expected, f"Multiplication failed: {result} != {expected}"
    print(f"[OK] multiply(7, 6) = {result}")

    # Test division
    result = divide(15, 3)
    expected = 5
    assert result == expected, f"Division failed: {result} != {expected}"
    print(f"[OK] divide(15, 3) = {result}")

    # Test division by zero
    try:
        divide(10, 0)
        print("[ERROR] Division by zero should have raised an error")
        return False
    except ZeroDivisionError as e:
        print(f"[OK] divide(10, 0) raised: {e}")

    # Test with floats
    result = add(2.5, 3.7)
    expected = 6.2
    assert abs(result - expected) < 0.0001, f"Float addition failed: {result} != {expected}"
    print(f"[OK] add(2.5, 3.7) = {result}")

    print("\n[PASSED] All tests passed!")
    return True

def main():
    """Main function to demonstrate calculator usage."""
    print("Calculator Demo")
    print("-" * 50)

    # Demonstrate basic operations
    a, b = 20, 5

    print(f"a = {a}, b = {b}")
    print(f"a + b = {add(a, b)}")
    print(f"a - b = {subtract(a, b)}")
    print(f"a * b = {multiply(a, b)}")
    print(f"a / b = {divide(a, b)}")

    print("\n" + "-" * 50)

    # Show division by zero handling
    print("Testing division by zero protection:")
    try:
        result = divide(a, 0)
    except ZeroDivisionError as e:
        print(f"Caught expected error: {e}")

if __name__ == "__main__":
    # Run tests first
    if test_calculator():
        print("\n" + "=" * 50)
        main()