functional_style.py

from dataclasses import dataclass


@dataclass
class UnitQuantity:
    value: int


@dataclass
class KilogramQuantity:
    value: float


OrderQuantity = UnitQuantity | KilogramQuantity

an_order_qty_in_units = UnitQuantity(10)
an_order_qty_in_kg = KilogramQuantity(2.5)


def quantity_label(oq: OrderQuantity) -> str:
    match (oq):
        case UnitQuantity(n):
            return f"{n} units"
        case KilogramQuantity(n):
            return f"{n} kg"


label = quantity_label(an_order_qty_in_kg)
print(label)