Properties: Controlled Attributes

Why this matters

Attributes are convenient, but plain ones cannot protect themselves: anyone can set age = -5. A property looks like an attribute from the outside (obj.age) while running code underneath, so you can validate, compute values on demand, or make something read-only, without changing how other code uses the class.

The idea

class Person:
    def __init__(self, age):
        self.age = age                   # goes through the setter below

    @property                            # the "getter": runs on  p.age
    def age(self):
        return self._age

    @age.setter                          # runs on  p.age = 30
    def age(self, value):
        if value < 0:
            raise ValueError("age cannot be negative")
        self._age = value

    @property                            # no setter: read-only
    def is_adult(self):
        return self._age >= 18

• The real value hides in self._age. A leading underscore means “internal, please do not touch”.
• Assigning in __init__ through self.age = age means even the first value is validated.
• A property with no setter is read-only: assigning raises AttributeError.
• Properties computed from other attributes never go out of date.

Try it

Create a Person, set p.age = -3 and read the ValueError. Then try p.is_adult = True.

Your turn

Write two classes:

• Temperature(celsius) with properties:
  • celsius: a normal readable and writable value that raises ValueError mentioning absolute zero for anything below -273.15 (also when creating the object).
  • fahrenheit: readable and writable; setting it changes the celsius value (and is validated too).
  • kelvin: read-only, celsius plus 273.15.
• Rectangle(width, height) with ordinary width and height attributes and read-only computed properties area, perimeter and is_square.

Recap

• @property makes a method behave like an attribute; @name.setter handles assignment.
• Use setters to validate and getters to compute.
• No setter means read-only.

Common mistakes

• Setting self.celsius inside the celsius setter, which calls the setter again forever (RecursionError). Store in self._celsius.
• Calling a property with parentheses (t.kelvin()): TypeError: 'float' object is not callable.
• Putting the @property method after the setter, so the setter has no property to attach to.