__contains__ — Intermediate Examples

# Real-world __contains__ usage
class Money:
    def __init__(self, amount, currency="USD"):
        self.amount = amount
        self.currency = currency

    def __repr__(self):
        return f"Money({self.amount}, '{self.currency}')"

    def __str__(self):
        symbols = {"USD": "$", "EUR": "€", "GBP": "£"}
        sym = symbols.get(self.currency, self.currency)
        return f"{sym}{self.amount:.2f}"

    def __add__(self, other):
        if self.currency != other.currency:
            raise ValueError(f"Cannot add {self.currency} and {other.currency}")
        return Money(self.amount + other.amount, self.currency)

    def __eq__(self, other):
        return self.amount == other.amount and self.currency == other.currency

    def __lt__(self, other):
        if self.currency != other.currency:
            raise ValueError("Cannot compare different currencies")
        return self.amount < other.amount

    def __bool__(self):
        return self.amount != 0

a = Money(10.50)
b = Money(5.25)
print(f"{a} + {b} = {a + b}")
print(f"Equal: {a == b}")
print(f"Less than: {b < a}")
print(f"Has value: {bool(a)}")

# __contains__ with NotImplemented
class Vector2D:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __add__(self, other):
        if not isinstance(other, Vector2D):
            return NotImplemented
        return Vector2D(self.x + other.x, self.y + other.y)

    def __mul__(self, scalar):
        if not isinstance(scalar, (int, float)):
            return NotImplemented
        return Vector2D(self.x * scalar, self.y * scalar)

    def __rmul__(self, scalar):
        return self.__mul__(scalar)

    def __repr__(self):
        return f"Vector2D({self.x}, {self.y})"

v = Vector2D(1, 2)
print(v + Vector2D(3, 4))
print(v * 3)
print(3 * v)  # Works because of __rmul__

try:
    v + 5  # Returns NotImplemented -> TypeError
except TypeError as e:
    print(f"TypeError: {e}")