{"id":2574,"date":"2021-10-17T07:31:52","date_gmt":"2021-10-17T07:31:52","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2574"},"modified":"2025-03-31T10:10:31","modified_gmt":"2025-03-31T10:10:31","slug":"python-readonly-property","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-readonly-property\/","title":{"rendered":"Python Readonly Property"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to define Python readonly property and how to use it to define computed properties.<\/p>\n\n\n\n

Introduction to the Python readonly property #<\/a><\/h2>\n\n\n\n

To define a readonly property<\/a>, you need to create a property with only the getter. However, it is not truly read-only because you can always access the underlying attribute and change it.<\/p>\n\n\n\n

The read-only properties are useful in some cases such as for computed properties.<\/p>\n\n\n\n

The following example defines a class called Circle<\/code> that has a radius<\/code> attribute and an area()<\/code> method:<\/p>\n\n\n

import<\/span> math\n\n\nclass<\/span> Circle<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, radius)<\/span>:<\/span>\n        self.radius = radius\n\n    def<\/span> area<\/span>(self)<\/span>:<\/span>\n        return<\/span> math.pi * self.radius ** 2<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
And the following creates a new Circle<\/code> object and returns its area:<\/p>\n\n\n
c = Circle(10<\/span>)\nprint(c.area())<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
314.1592653589793<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Here’s the complete script:<\/p>\n\n\n
import math\n\n\nclass Circle:\n    def __init__(self, radius):\n        self.radius = radius\n\n    def area(self):\n        return math.pi * self.radius ** 2\n    \nc = Circle(10)\nprint(c.area())    <\/code><\/span><\/pre>\n\n\n
Try it<\/a><\/p>\n\n\n\n
Output:<\/p>\n\n\n
314.1592653589793<\/span><\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
This code works perfectly fine.<\/p>\n\n\n\n
import<\/span> math\n\n\nclass<\/span> Circle<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, radius)<\/span>:<\/span>\n        self.radius = radius\n\n    @property<\/span>\n    def<\/span> area<\/span>(self)<\/span>:<\/span>\n        return<\/span> math.pi * self.radius ** 2<\/span>\n\n\nc = Circle(10<\/span>)\nprint(c.area)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The area is calculated from the radius<\/code> attribute. Therefore, it’s often called a calculated or computed property.<\/p>\n\n\n\n
Caching calculated properties #<\/a><\/h2>\n\n\n\n
Suppose you create a new circle object and access the area property many times. Each time, the area needs to be recalculated, which is not efficient.<\/p>\n\n\n\n
To make it more performant, you need to recalculate the area of the circle only when the radius changes. If the radius doesn’t change, you can reuse the previously calculated area.<\/p>\n\n\n\n
To do it, you can use the caching technique:<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
But it would be more natural that the area is a property of the Circle<\/code> object, not a method. To make the area()<\/code> method as a property of the Circle<\/code> class, you can use the @property<\/code> decorator<\/a> as follows:<\/p>\n\n\n