Operator Overloading

Part of the Object Oriented Programming section of Coddy's Python journey — lesson 34 of 64.

Operator overloading allows your classes to work with Python's built-in operators (+, -, *, etc.) by implementing special magic methods.

Here is an example of a class with operator overloading:

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    
    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)
    
    def __str__(self):
        return f"Vector({self.x}, {self.y})"

The __add__ method defines what happens when you use the + operator:

v1 = Vector(2, 3)
v2 = Vector(5, 7)
result = v1 + v2  # Calls v1.__add__(v2)
print(result)

The __mul__ method defines what happens when you use the * operator:

v1 = Vector(2, 3)
scaled = v1 * 3   # Calls v1.__mul__(3)
print(scaled)

Output:

Vector(7, 10)
Vector(6, 9)

Add comparison operators:

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    
    def __eq__(self, other):
        return self.x == other.x and self.y == other.y
    
    def __str__(self):
        return f"Vector({self.x}, {self.y})"

v1 = Vector(2, 3)
v2 = Vector(2, 3)
v3 = Vector(1, 1)

print(v1 == v2)  # True - calls v1.__eq__(v2)
print(v1 == v3)  # False

Key Point: Operator overloading uses magic methods like __add__ (+), __sub__ (-), __mul__ (*), __eq__ (==) to define how operators work with your objects. This makes your classes behave naturally with Python's built-in operators.

Challenge

Medium

In this challenge, you'll implement a Money class that represents monetary amounts with robust operator overloading. Your implementation will be thoroughly tested against a comprehensive test suite.

money.py - This is the only file you need to edit. It contains the class definition with TODO comments guiding your implementation.
driver.py - Contains extensive test scenarios that validate your implementation (do not modify).

Implement the Money class with the following features:

Constructor that takes amount (float) and currency (string)
Addition (+) of Money objects with the same currency
Multiplication (*) by a number to scale the amount
Equality comparison (==) between Money objects
String representation in the format “X.XX CUR”

Cheat sheet

Operator overloading allows classes to work with Python's built-in operators by implementing special magic methods:

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    
    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)
    
    def __eq__(self, other):
        return self.x == other.x and self.y == other.y
    
    def __str__(self):
        return f"Vector({self.x}, {self.y})"

Common magic methods for operator overloading:

__add__ for + operator
__sub__ for - operator
__mul__ for * operator
__eq__ for == operator
__str__ for string representation

Usage example:

v1 = Vector(2, 3)
v2 = Vector(5, 7)
result = v1 + v2  # Calls v1.__add__(v2)
scaled = v1 * 3   # Calls v1.__mul__(3)
print(v1 == v2)   # Calls v1.__eq__(v2)

Try it yourself

from money import Money

# Test case handler
test_case = input()

def test_basic_functionality():
    # Test initialization and string representation
    m1 = Money(10.0, "USD")
    assert str(m1) == "10.00 USD", f"__str__ method failed, got {str(m1)}"
    
    # Test addition
    m2 = Money(20.0, "USD")
    m3 = m1 + m2
    assert str(m3) == "30.00 USD", f"Addition failed, got {str(m3)}"
    
    # Test different currency addition
    m4 = Money(20.0, "EUR")
    try:
        m5 = m1 + m4
        assert False, "Adding different currencies should raise an error"
    except ValueError as e:
        assert str(e) == "Cannot add different currencies", f"Wrong error message: {str(e)}"
    
    # Test multiplication
    m6 = m1 * 3
    assert str(m6) == "30.00 USD", f"Multiplication failed, got {str(m6)}"
    
    # Test equality
    assert m1 == Money(10.0, "USD"), "Equality test failed"
    assert m1 != m2, "Inequality test failed"
    
    print("All basic functionality tests passed!")

def test_zero_values():
    # Test with zero amount
    m1 = Money(0.0, "USD")
    assert str(m1) == "0.00 USD", f"Zero amount string representation failed, got {str(m1)}"
    
    # Test addition with zero
    m2 = Money(10.0, "USD")
    m3 = m1 + m2
    assert str(m3) == "10.00 USD", f"Addition with zero failed, got {str(m3)}"
    
    # Test multiplication by zero
    m4 = m2 * 0
    assert str(m4) == "0.00 USD", f"Multiplication by zero failed, got {str(m4)}"
    
    # Test equality with zero amount
    assert m1 == Money(0.0, "USD"), "Equality with zero amount failed"
    assert m1 != m2, "Inequality with zero amount failed"
    
    print("All zero value tests passed!")

def test_negative_values():
    # Test with negative amount
    m1 = Money(-10.0, "USD")
    assert str(m1) == "-10.00 USD", f"Negative amount string representation failed, got {str(m1)}"
    
    # Test addition with negative amounts
    m2 = Money(20.0, "USD")
    m3 = m1 + m2
    assert str(m3) == "10.00 USD", f"Addition with negative amount failed, got {str(m3)}"
    
    m4 = Money(-5.0, "USD")
    m5 = m1 + m4
    assert str(m5) == "-15.00 USD", f"Addition of two negative amounts failed, got {str(m5)}"
    
    # Test multiplication by negative scalar
    m6 = m2 * -2
    assert str(m6) == "-40.00 USD", f"Multiplication by negative scalar failed, got {str(m6)}"
    
    # Test equality with negative amounts
    assert m1 == Money(-10.0, "USD"), "Equality with negative amount failed"
    assert m1 != m2, "Inequality with negative amount failed"
    
    print("All negative value tests passed!")

def test_large_values():
    # Test with very large amounts
    m1 = Money(1000000.0, "USD")
    assert str(m1) == "1000000.00 USD", f"Large amount string representation failed, got {str(m1)}"
    
    # Test addition with large amounts
    m2 = Money(2000000.0, "USD")
    m3 = m1 + m2
    assert str(m3) == "3000000.00 USD", f"Addition with large amounts failed, got {str(m3)}"
    
    # Test multiplication with large scalar
    m4 = m1 * 1000
    assert str(m4) == "1000000000.00 USD", f"Multiplication with large scalar failed, got {str(m4)}"
    
    print("All large value tests passed!")

def test_precision():
    # Test with fractional amounts
    m1 = Money(10.25, "USD")
    assert str(m1) == "10.25 USD", f"Fractional amount string representation failed, got {str(m1)}"
    
    m2 = Money(10.2, "USD")
    assert str(m2) == "10.20 USD", f"Two decimal place formatting failed, got {str(m2)}"
    
    # Test addition with fractional amounts
    m3 = Money(0.75, "USD")
    m4 = m1 + m3
    assert str(m4) == "11.00 USD", f"Addition with fractional amounts failed, got {str(m4)}"
    
    # Test multiplication with fractional scalar
    m5 = m1 * 0.5
    assert str(m5) == "5.13 USD", f"Multiplication with fractional scalar failed, got {str(m5)}"
    
    print("All precision tests passed!")

def test_type_validation():
    try:
        # These operations should work without errors
        m1 = Money(10.0, "USD")
        m2 = m1 * 2
        m3 = m1 * 2.5
        
        # Test equality with different types
        assert (m1 == "10.00 USD") == False, "Equality with string should return False"
        assert (m1 == 10.0) == False, "Equality with number should return False"
        
        print("All type validation tests passed!")
    except Exception as e:
        print(f"Type validation test failed: {e}")

def test_currency_case_sensitivity():
    # Test currency case sensitivity
    m1 = Money(10.0, "USD")
    m2 = Money(10.0, "usd")
    
    # Currencies should be case-sensitive
    assert m1 != m2, "Currency comparison should be case-sensitive"
    
    # Adding different cases should fail
    try:
        m3 = m1 + m2
        assert False, "Adding different currency cases should raise an error"
    except ValueError as e:
        assert str(e) == "Cannot add different currencies", f"Wrong error message: {str(e)}"
    
    print("All currency case sensitivity tests passed!")

def test_performance():
    # Create many Money objects and perform operations
    base = Money(1.0, "USD")
    result = base
    
    # Perform 1000 additions
    for i in range(1000):
        result = result + Money(1.0, "USD")
    
    assert str(result) == "1001.00 USD", f"Performance test addition failed, got {str(result)}"
    
    # Perform 10 multiplications
    result = base
    for i in range(10):
        result = result * 2
    
    assert str(result) == "1024.00 USD", f"Performance test multiplication failed, got {str(result)}"
    
    print("All performance tests passed!")

# Run the appropriate test based on input
if test_case == "basic_test":
    test_basic_functionality()
elif test_case == "zero_values":
    test_zero_values()
elif test_case == "negative_values":
    test_negative_values()
elif test_case == "large_values":
    test_large_values()
elif test_case == "precision":
    test_precision()
elif test_case == "type_validation":
    test_type_validation()
elif test_case == "currency_case":
    test_currency_case_sensitivity()
elif test_case == "performance":
    test_performance()
else:
    print(f"Unknown test case: {test_case}")