Python Class Constructors: Control Your Object Instantiation


Class constructors are a fundamental part of object-oriented programming in Python. They allow you to create and properly initialize objects of a given class, making those objects ready to use. Class constructors internally trigger Python’s instantiation process, which runs through two main steps: instance creation and instance initialization.

If you want to dive deeper into how Python internally constructs objects and learn how to customize the process, then this tutorial is for you.

In this tutorial, you’ll:

Understand Python’s internal instantiation process
Customize object initialization using .__init__()
Fine-tune object creation by overriding .__new__()

With this knowledge, you’ll be able to tweak the creation and initialization of objects in your custom Python classes, which will give you control over the instantiation process at a more advanced level.

To better understand the examples and concepts in this tutorial, you should be familiar with object-oriented programming and special methods in Python.

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Python’s Class Constructors and the Instantiation Process

Like many other programming languages, Python supports object-oriented programming. At the heart of Python’s object-oriented capabilities, you’ll find the class keyword, which allows you to define custom classes that can have attributes for storing data and methods for providing behaviors.

Once you have a class to work with, then you can start creating new instances or objects of that class, which is an efficient way to reuse functionality in your code.

Creating and initializing objects of a given class is a fundamental step in object-oriented programming. This step is often referred to as object construction or instantiation. The tool responsible for running this instantiation process is commonly known as a class constructor.

Getting to Know Python’s Class Constructors

In Python, to construct an object of a given class, you just need to call the class with appropriate arguments, as you would call any function:

>>>>>> class SomeClass:

>>> # Call the class to construct an object
>>> SomeClass()
<__main__.SomeClass object at 0x7fecf442a140>

In this example, you define SomeClass using the class keyword. This class is currently empty because it doesn’t have attributes or methods. Instead, the class’s body only contains a pass statement as a placeholder statement that does nothing.

Then you create a new instance of SomeClass by calling the class with a pair of parentheses. In this example, you don’t need to pass any argument in the call because your class doesn’t take arguments yet.

In Python, when you call a class as you did in the above example, you’re calling the class constructor, which creates, initializes, and returns a new object by triggering Python’s internal instantiation process.

A final point to note is that calling a class isn’t the same as calling an instance of a class. These are two different and unrelated topics. To make a class’s instance callable, you need to implement a .__call__() special method, which has nothing to do with Python’s instantiation process.

Understanding Python’s Instantiation Process

You trigger Python’s instantiation process whenever you call a Python class to create a new instance. This process runs through two separate steps, which you can describe as follows:

Create a new instance of the target class
Initialize the new instance with an appropriate initial state

To run the first step, Python classes have a special method called .__new__(), which is responsible for creating and returning a new empty object. Then another special method, .__init__(), takes the resulting object, along with the class constructor’s arguments.

The .__init__() method takes the new object as its first argument, self. Then it sets any required instance attribute to a valid state using the arguments that the class constructor passed to it.

In short, Python’s instantiation process starts with a call to the class constructor, which triggers the instance creator, .__new__(), to create a new empty object. The process continues with the instance initializer, .__init__(), which takes the constructor’s arguments to initialize the newly created object.

To explore how Python’s instantiation process works internally, consider the following example of a Point class that implements a custom version of both methods, .__new__() and .__init__(), for demonstration purposes:

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