from atom.api import Atom, Str, Range, Bool, observe


class Person(Atom):
    """ A simple class representing a person object.

    """
    last_name = Str()

    first_name = Str()

    age = Range(low=0)

    debug = Bool(False)

    @observe('age')
    def debug_print(self, change):
        """ Prints out a debug message whenever the person's age changes.

        """
        if self.debug:
            templ = "{first} {last} is {age} years old."
            s = templ.format(
                first=self.first_name, last=self.last_name, age=self.age,
            )
            print(s)

    def _default_first_name(self):
        return 'John'


john = Person(last_name='Doe', age=42)
john.debug = True
john.age = 43  # prints message
john.age = 'forty three'   # raises TypeError

from atom.api import Atom, observe

class InventoryItem(Atom):
    """Class for keeping track of an item in inventory."""

    name: str
    unit_price: float
    quantity_on_hand: int = 0

    def total_cost(self) -> float:
        return self.unit_price * self.quantity_on_hand

    @observe("unit_price")
    def check_for_price_reduction(self, change):
        savings = change.get("oldvalue", 0) - change.get("value")
        if savings > 0:
            print(f"Save ${savings} now on {self.name}s!")

>>> w = InventoryItem(name="widget", unit_price=1.99, quantity_on_hand=10)
>>> w.unit_price = 1.00
Save $0.99 now on widgets!