16_property.py

# property: attribute syntax with method behavior — the pythonic
# alternative to get_x()/set_x() pairs.


class Temperature:
    def __init__(self, celsius):
        self._celsius = celsius  # underscore: "internal, use the property"

    @property
    def celsius(self):
        return self._celsius

    @celsius.setter
    def celsius(self, value):
        if value < -273.15:
            raise ValueError("below absolute zero")
        self._celsius = value

    # A derived, read-only attribute: computed on access, no setter at all
    @property
    def fahrenheit(self):
        return self._celsius * 9 / 5 + 32


t = Temperature(25)
print(t.celsius, t.fahrenheit)  # reads look like plain attributes

t.celsius = 30  # goes through the setter
print(t.fahrenheit)

try:
    t.celsius = -300  # setter validates
except ValueError as e:
    print("rejected:", e)

try:
    t.fahrenheit = 100  # no setter defined -> AttributeError
except AttributeError as e:
    print("read-only:", e)