properties.md

Python by Example: Properties

The @property decorator lets you access a method like an attribute. Use it to add validation or computed values without changing the public interface of a class. Pair it with @name.setter to intercept assignments.

What you'll learn:

• @property for computed read access
• @name.setter for validated assignment
• Keeping implementation details private with _name

class Temperature:
    def __init__(self, celsius):
        self._celsius = celsius

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

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

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


t = Temperature(100)
print(t.celsius)
print(t.fahrenheit)

t.celsius = 0
print(t.celsius)
print(t.fahrenheit)

fahrenheit is a computed property with no setter, so it's read-only. The leading underscore on _celsius signals it's an internal detail; callers use the celsius property instead. The call site t.celsius looks like plain attribute access—no parentheses needed.

To run this program:

$ python source/properties.py
100
212.0
0
32.0

Tip: Add @property gradually—start with plain attributes and add a property only when you need validation or computation. The call site stays the same either way.

Try it: Add a kelvin property that converts Celsius to Kelvin (add 273.15).

Source: properties.py

Next: Dataclasses