Advanced Properties

Property decorators, getters/setters, and computed properties.

Overview

Master property patterns.

Property Decorators

class Circle:
    def __init__(self, radius):
        self._radius = radius
    
    @property
    def radius(self):
        return self._radius
    
    @radius.setter
    def radius(self, value):
        if value < 0:
            raise ValueError("Radius cannot be negative")
        self._radius = value
    
    @property
    def area(self):
        return 3.14159 * self._radius ** 2
    
    @property
    def circumference(self):
        return 2 * 3.14159 * self._radius

c = Circle(5)
print(c.area)  # 78.53975
c.radius = 10
print(c.area)  # 314.159

Computed Properties

class Temperature:
    def __init__(self, celsius=0):
        self._celsius = celsius
    
    @property
    def celsius(self):
        return self._celsius
    
    @celsius.setter
    def celsius(self, value):
        self._celsius = value
    
    @property
    def fahrenheit(self):
        return self._celsius * 9/5 + 32
    
    @fahrenheit.setter
    def fahrenheit(self, value):
        self._celsius = (value - 32) * 5/9
    
    @property
    def kelvin(self):
        return self._celsius + 273.15
    
    @kelvin.setter
    def kelvin(self, value):
        self._celsius = value - 273.15

t = Temperature(100)
print(t.fahrenheit)  # 212.0
print(t.kelvin)  # 373.15

Cached Properties

from functools import cached_property

class DataAnalyzer:
    def __init__(self, data):
        self.data = data
    
    @cached_property
    def mean(self):
        print("Computing mean...")
        return sum(self.data) / len(self.data)
    
    @cached_property
    def std_dev(self):
        print("Computing std_dev...")
        variance = sum((x - self.mean) ** 2 for x in self.data) / len(self.data)
        return variance ** 0.5

analyzer = DataAnalyzer([1, 2, 3, 4, 5])
print(analyzer.mean)  # Computes
print(analyzer.mean)  # Uses cached value
print(analyzer.std_dev)  # Computes (uses cached mean)

Practice

Implement a property-based validation system.