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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:
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. Python’s Class Constructors and the Instantiation ProcessLike many other programming languages, Python supports object-oriented programming. At the heart of Python’s object-oriented capabilities, you’ll find the
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 ConstructorsIn 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: >>>
In this example, you define Then you create a new instance of 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 Understanding Python’s Instantiation ProcessYou 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:
To run the first step, Python classes have a special method called The In short, Python’s instantiation process starts with a call to the class constructor, which triggers the instance creator, To explore how Python’s instantiation process works internally, consider the following example of a
Here’s a breakdown of what this code does:
With >>>
Calling the In this example, the call to the constructor also lets you know the steps that Python internally runs to construct the instance.
First, Python calls To continue learning about class instantiation in Python, you can try running both steps manually: >>>
In this example, you first call Once you have the new object, then you can initialize it by calling A subtle and important detail to note about Consider the following example, in which the
Because >>>
The call to the Now that you know the steps
that Python internally takes to create instances of a given class, you’re ready to dig a little deeper into other characteristics of Object Initialization With .__init__()In Python, the The purpose of this initialization step is to leave your new objects in a valid state so that you can start using them right away in your code. In this section, you’ll learn the basics of writing your own Providing Custom Object InitializersThe most bare-bones implementation of >>>
As you learned before, It’s important to note that, without counting Additionally, keep in mind that >>>
In this example, the The error message in the above example says that With the above
implementation of In >>>
In this updated implementation of Now say that you’re using
inheritance to create a custom class hierarchy and reuse some functionality in your code. If your subclasses provide a >>>
The first line in the To wrap up this section, you should know that the base implementation of Building Flexible Object InitializersYou can
make your objects’ initialization step flexible and versatile by tweaking the As a quick example,
check out the following
In this example, The class’s final behavior will depend on
the value of To try >>>
In the first example, you create an In the second example, you use a Even though this is a toy example, it showcases how default argument values are a powerful Python feature that you can use to write flexible initializers for your classes. These initializers will allow you to instantiate your classes using different sets of arguments depending on your needs. Okay! Now that you know the basics of Object Creation With .__new__()When writing Python classes, you typically don’t need to provide your own implementation of the However, there are a few interesting use cases for this method. For
example, you can use In the following sections, you’ll learn how to write custom implementations of Providing Custom Object CreatorsTypically, you’ll write a custom implementation of
With these three succinct steps, you’ll be able to customize the instance creation step in the Python instantiation process. Here’s an example of how you can translate these steps into Python code:
This example provides a sort of template implementation of Note that you’re using In the first line of The next step is to customize your newly created instance. You can do whatever you need to do to customize the instance at hand. Finally, in the third step, you need to return the new instance to continue the instantiation process with the initialization step. It’s important to
note that >>>
In this example, you hand over However, >>>
In this implementation of Cool! Now that you know the basics of writing your own implementations of Subclassing Immutable Built-in TypesTo kick things off, you’ll start with a use case of Here’s a first approach to
this problem, using the >>>
When you subclass an immutable built-in data type, you get an error. Part of the problem is that the value is set during creation, and it’s too late to change it during initialization. Additionally, To work
around this issue, you can initialize the object at creation time with >>>
In this example, That’s it! Now your Returning Instances of a Different ClassReturning an object of a different
class is a requirement that can raise the need for a custom implementation of Check out the following example, in which the
In this example, Here’s how you can use this >>>
Every time you instantiate Finally, note how the Allowing Only a Single Instance in Your ClassesSometimes you need to implement a class that allows the creation of a single instance only. This type of class is commonly known as a singleton class. In this situation, the Here’s an example of coding a >>>
The If this condition is true, then the Then you instantiate Partially Emulating collections.namedtupleAs a final example of how to take advantage of The code below implements a
Here’s how this factory function works line by line:
To try your >>>
In this code snippet, you create a new Then you create a Because tuples are immutable data types in Python, you can’t assign new values to the point’s coordinates in place. If you try to do that, then you get an Finally,
calling ConclusionNow you know how Python class constructors allow you to instantiate classes, so you can create concrete and ready-to-use objects in your code. In Python,
class constructors internally trigger the instantiation or construction process, which goes through instance creation and instance initialization. These steps are driven by the By learning about Python’s class constructors, the instantiation process, and the In this tutorial, you learned:
Now you’re ready to take advantage of this knowledge to fine-tune your class constructors and take full control over instance creation and initialization in your object-oriented programming adventure with Python. Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Using Python Class Constructors Which method is automatically executed when an instance of a class is created in memory?The __init__ special method is automatically executed when an instance of class is created. It is also called class constructor.
What type of programming contains class definitions?In object-oriented programming , a class is a template definition of the method s and variable s in a particular kind of object . Thus, an object is a specific instance of a class; it contains real values instead of variables. The class is one of the defining ideas of object-oriented programming.
What method in a class initializes an object's data attributes?An __init__ method is a special operation that defines how to initialize an object. It's called when an object is created. You can define other methods (for example, functions inside a class) to do other operations. When using a class, the dot notation on an object accesses data attributes and methods.
What are the procedures that an object performs called?The procedures that an object performs are known as METHODS. Methods are functions that perform operations on the object's data attributes. The object is a self-contained unit consisting of data attributes and methods. Thus, code and data is not separated - this is called ENCAPSULATION.
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