@property in Python: getters, setters and validation

@property lets a method look like a normal attribute. You read c.area without brackets, and Python runs a method to get the value. With a setter, you can also run code when someone assigns a new value, for example to reject a wrong one.

The program

python

import math


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

    @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 math.pi * self._radius ** 2


c = Circle(5)
print(f"radius {c.radius}, area {c.area:.2f}")

c.radius = 10
print(f"radius {c.radius}, area {c.area:.2f}")

try:
    c.radius = -3
except ValueError as e:
    print("Error:", e)
print("radius is still", c.radius)

output

radius 5, area 78.54
radius 10, area 314.16
Error: radius cannot be negative
radius is still 10

How it works

The real value is stored in self._radius. The underscore at the start tells other programmers "this is internal, use radius instead".
@radius.setter runs on every assignment to c.radius, including the one in __init__. So Circle(-1) is rejected too.
area has no setter, so it is read-only. It is calculated every time you read it, so it is always correct for the current radius. If area were a normal attribute set in __init__, it would keep the old value after c.radius = 10.
math.pi is more accurate than typing 3.14159.

Read-only attributes

Assigning to a property that has no setter raises AttributeError:

python

import math

class Circle:
    def __init__(self, radius):
        self.radius = radius

    @property
    def area(self):
        return math.pi * self.radius ** 2

c = Circle(2)
try:
    c.area = 100
except AttributeError as e:
    print("Error:", e)

output

Error: property 'area' of 'Circle' object has no setter

Property decorator in Python

The program

How it works

Read-only attributes