Properties and Class Methods

The `@property` decorator

@property turns a method into an attribute-style accessor. Callers use obj.celsius instead of obj.get_celsius(), keeping the public API clean while allowing validation logic to run behind the scenes.

class Temperature:
    def __init__(self, celsius: float = 0.0) -> None:
        self._celsius = celsius    # underscore = internal storage

    @property
    def celsius(self) -> float:
        return self._celsius

Setters

Pair @property with @name.setter to intercept assignment:

    @celsius.setter
    def celsius(self, value: float) -> None:
        if value < -273.15:
            raise ValueError("Temperature below absolute zero!")
        self._celsius = value

Now t.celsius = -300 raises ValueError automatically. Without a setter, the property is read-only — attempts to assign raise AttributeError.

Computed (derived) properties

A property can compute its value on the fly with no backing storage:

    @property
    def fahrenheit(self) -> float:
        return self._celsius * 9 / 5 + 32

t.fahrenheit looks like a plain attribute to the caller, but it is recalculated each time.

`@classmethod` — alternative constructors

A class method receives the class (cls) as its first argument instead of the instance. The most common use is providing alternative constructors:

    @classmethod
    def from_fahrenheit(cls, f: float) -> "Temperature":
        return cls((f - 32) * 5 / 9)

boiling = Temperature.from_fahrenheit(212)
print(boiling.celsius)   # 100.0

Subclasses automatically inherit the class method and cls will refer to the subclass — so the factory creates an instance of the correct type.

`@staticmethod` — utility functions

A static method receives neither the instance nor the class. Use it for utilities that logically belong to the class but do not need access to class or instance state:

    @staticmethod
    def absolute_zero() -> float:
        return -273.15

print(Temperature.absolute_zero())   # -273.15

Full runnable demo

Open in an online REPL

class Temperature:
    def __init__(self, celsius: float = 0.0) -> None:
        self._celsius = celsius

    @property
    def celsius(self) -> float:
        return self._celsius

    @celsius.setter
    def celsius(self, value: float) -> None:
        if value < -273.15:
            raise ValueError("Temperature below absolute zero!")
        self._celsius = value

    @property
    def fahrenheit(self) -> float:
        return self._celsius * 9 / 5 + 32

    @classmethod
    def from_fahrenheit(cls, f: float) -> "Temperature":
        return cls((f - 32) * 5 / 9)

    @staticmethod
    def absolute_zero() -> float:
        return -273.15


t = Temperature(100)
print(t.celsius)
print(t.fahrenheit)
t.celsius = 0
print(t.fahrenheit)
t2 = Temperature.from_fahrenheit(212)
print(t2.celsius)
print(Temperature.absolute_zero())