Python's property(): Add Managed Attributes to Your Classes :

Python's property(): Add Managed Attributes to Your Classes
by:
blow post content copied from  Real Python
click here to view original post

With Python’s property(), you can create managed attributes in your classes. You can use managed attributes when you need to modify an attribute’s internal implementation and don’t want to change the class’s public API. Providing stable APIs will prevent you from breaking your users’ code when they rely on your code.

Properties are arguably the most popular way to create managed attributes quickly and in the purest Pythonic style.

In this tutorial, you’ll learn how to:

• Create managed attributes or properties in your classes
• Perform lazy attribute evaluation and provide computed attributes
• Make your classes Pythonic using properties instead of setter and getter methods
• Create read-only and read-write properties
• Create consistent and backward-compatible APIs for your classes

You’ll also write practical examples that use property() for validating input data, computing attribute values dynamically, logging your code, and more. To get the most out of this tutorial, you should know the basics of object-oriented programming, classes, and decorators in Python.

Get Your Code: Click here to download the free sample code that shows you how to use Python’s property() to add managed attributes to your classes.

Take the Quiz: Test your knowledge with our interactive “Python's property(): Add Managed Attributes to Your Classes” quiz. You’ll receive a score upon completion to help you track your learning progress:

---

Interactive Quiz

Python's property(): Add Managed Attributes to Your Classes

In this quiz, you'll test your understanding of Python's property(). With this knowledge, you'll be able to create managed attributes in your classes, perform lazy attribute evaluation, provide computed attributes, and more.

Managing Attributes in Your Classes

When you define a class in an object-oriented programming language, you’ll probably end up with some instance and class attributes. In other words, you’ll end up with variables that are accessible through the instance, class, or even both, depending on the language. Attributes represent and hold the internal state of a given object, which you’ll often need to access and mutate.

Typically, you have at least two ways to access and mutate an attribute. Either you can access and mutate the attribute directly or you can use methods. Methods are functions attached to a given class. They provide the behaviors and actions that an object can perform with its internal data and attributes.

If you expose attributes to the user, then they become part of the class’s public API. This means that your users will access and mutate them directly in their code. The problem comes when you need to change the internal implementation of a given attribute.

Say you’re working on a Circle class and add an attribute called .radius, making it public. You finish coding the class and ship it to your end users. They start using Circle in their code to create a lot of awesome projects and applications. Good job!

Now suppose that you have an important user that comes to you with a new requirement. They don’t want Circle to store the radius any longer. Instead, they want a public .diameter attribute.

At this point, removing .radius to start using .diameter could break the code of some of your other users. You need to manage this situation in a way other than removing .radius.

Programming languages such as Java and C++ encourage you to never expose your attributes to avoid this kind of problem. Instead, you should provide getter and setter methods, also known as accessors and mutators, respectively. These methods offer a way to change the internal implementation of your attributes without changing your public API.

Note: Getter and setter methods are often considered an anti-pattern and a signal of poor object-oriented design. The main argument behind this proposition is that these methods break encapsulation. They allow you to access and mutate the components of your objects from the outside.

These programming languages need getter and setter methods because they don’t have a suitable way to change an attribute’s internal implementation when a given requirement changes. Changing the internal implementation would require an API modification, which can break your end users’ code.

The Getter and Setter Approach in Python

Technically, there’s nothing that stops you from using getter and setter methods in Python. Here’s a quick example that shows how this approach would look:

Python point_v1.py

class Point:
    def __init__(self, x, y):
        self._x = x
        self._y = y

    def get_x(self):
        return self._x

    def set_x(self, value):
        self._x = value

    def get_y(self):
        return self._y

    def set_y(self, value):
        self._y = value

In this example, you create a Point class with two non-public attributes ._x and ._y to hold the Cartesian coordinates of the point at hand.

Note: Python doesn’t have the notion of access modifiers, such as private, protected, and public, to restrict access to attributes and methods. In Python, the distinction is between public and non-public class members.

If you want to signal that a given attribute or method is non-public, then you have to use the well-known Python convention of prefixing the name with an underscore (_). That’s the reason behind the naming of the attributes ._x and ._y.

Note that this is just a convention. It doesn’t stop you and other programmers from accessing the attributes using dot notation, as in obj._attr. However, it’s bad practice to violate this convention.

To access and mutate the value of either ._x or ._y, you can use the corresponding getter and setter methods. Go ahead and save the above definition of Point in a Python module and import the class into an interactive session. Then run the following code:

Python

>>> from point_v1 import Point

>>> point = Point(12, 5)
>>> point.get_x()
12
>>> point.get_y()
5

>>> point.set_x(42)
>>> point.get_x()
42

>>> # Non-public attributes are still accessible
>>> point._x
42
>>> point._y
5

With .get_x() and .get_y(), you can access the current values of ._x and ._y. You can use the setter method to store a new value in the corresponding managed attribute. From the two final examples, you can confirm that Python doesn’t restrict access to non-public attributes. Whether or not you access them directly is up to you.

Read the full article at https://realpython.com/python-property/ »

---

[ Improve Your Python With 🐍 Python Tricks 💌 – Get a short & sweet Python Trick delivered to your inbox every couple of days. >> Click here to learn more and see examples ]

October 21, 2024 at 07:30PM
Click here for more details...

=============================
The original post is available in Real Python by
this post has been published as it is through automation. Automation script brings all the top bloggers post under a single umbrella.
The purpose of this blog, Follow the top Salesforce bloggers and collect all blogs in a single place through automation.
============================