{"id":78094,"date":"2020-04-07T15:16:56","date_gmt":"2020-04-07T09:46:56","guid":{"rendered":"https:\/\/techvidvan.com\/tutorials\/?p=78094"},"modified":"2024-08-22T18:10:25","modified_gmt":"2024-08-22T12:40:25","slug":"python-constructors","status":"publish","type":"post","link":"https:\/\/techvidvan.com\/tutorials\/python-constructors\/","title":{"rendered":"Python Constructors – Best Ways to Implement Constructors"},"content":{"rendered":"

In this article, we will explore the concept of constructors in Python, which are essential for initializing data attributes within a class. Before diving into constructors, it’s important to understand the basics of classes and objects in Python. A class serves as a blueprint for creating objects, encapsulating data and behavior together. Objects are instances of classes, representing specific entities with unique attributes and methods.<\/p>\n

Constructors provide a way to set up the initial state of an object when it is created, ensuring that all necessary attributes are properly initialized. By the end of this article, you’ll have a solid understanding of how to implement and utilize constructors in Python to build robust and maintainable code.<\/p>\n

Why use Python Constructors?<\/h3>\n

Before delving into constructors in python<\/strong>, we first need to get the hang of one important concept. That is, in Python<\/strong>, you can\u2019t<\/strong> use<\/strong> a variable<\/strong> before assigning<\/strong> it a value<\/strong>, no matter where you use<\/strong> the variable<\/strong>.<\/p>\n

To demonstrate the seriousness of this issue, consider this short session at the Python Shell<\/strong>.<\/p>\n

The first statement fails<\/strong> to execute<\/strong> when we define neither<\/strong> of the variables<\/strong>.<\/p>\n

>>> x += y<\/pre>\n
Output:<\/strong><\/p>\n
NameError: name \u2018x\u2019 is not defined<\/div>\n
\n
The interpreter complains that \u2018x\u2019<\/strong> is not defined<\/strong>!<\/p>\n
Let\u2019s now define \u2018x\u2019 and then see what the interpreter has to say.<\/p>\n<\/div>\n
>>> x = 10\r\n>>> x += y\r\n<\/pre>\n
Output:<\/strong><\/p>\n
NameError: name \u2018y\u2019 is not defined<\/div>\n
The interpreter complains again! This time it says \u2018y\u2019<\/strong> is not defined<\/strong>.<\/p>\n
Say no more interpreters! We\u2019ll define \u2018y\u2019 too for you.<\/p>\n
>>> y = 1\r\n>>> x += y\r\n>>> x\r\n11\r\n>>> y\r\n1\r\n<\/pre>\n
Voila<\/strong>! It works now.<\/p>\n
The bottom line is, no matter where you use variables<\/strong> in Python, you have to initialize<\/strong> them with a starting value<\/strong>.<\/p>\n
But the question here is: how do we do this for a new object created<\/strong> from a class?<\/p>\n
In object-oriented programming, \u201cconstructors<\/strong>\u201d take care of this situation. In other OOP languages, a constructor is a special method<\/strong> that is responsible for both object instantiation<\/strong> (i.e. creating a new object) and attribute initialization<\/strong> (i.e. assigning values to the attributes for that object).<\/p>\n
But in Python, the interpreter automatically handles the object instantiation<\/strong> part. So all you need a constructor for is attribute initialization.<\/strong><\/p>\n
So we now know the \u201cwhy\u201d<\/strong>, let\u2019s move on to the \u201chow<\/strong>\u201d!<\/p>\n
Coding Constructors<\/h3>\n
We said in the last section that a special method<\/strong> is responsible<\/strong> for assigning values<\/strong> to attributes of an object<\/strong>. This special method<\/strong>, or as we like to call in Python, this \u201cmagic method<\/strong>\u201d is the __init__ method<\/strong>.<\/p>\n
Let\u2019s add __init__<\/strong> to a class Student<\/strong> that has two attributes<\/strong>, name<\/strong> and roll number<\/strong>.<\/p>\n
For now, let the __init__<\/strong> method be empty<\/strong> with just a pass statement<\/strong> inside<\/strong> it (we\u2019ll add some behavior<\/strong> just in a moment).<\/p>\n
class Student:\r\n    def __init__(self):\r\n        pass\r\n    def show(self):\r\n        print(\"Name: \", self.name)\r\n        print(\"Roll no.: \", self.roll)\r\n<\/pre>\n
Note that like every other method defined in a class<\/strong>, the __init__<\/strong> method too needs \u201cself\u201d<\/strong> as its first parameter<\/strong>.<\/p>\n
Now, all we need is to use __init__<\/strong> to initialize<\/strong> our class\u2019s attributes<\/strong> (name<\/strong> and roll<\/strong>). The question is: where do we get these initialization values<\/strong> from and how do these values get into __init__<\/strong>?<\/p>\n
Passing arguments to __init__<\/h3>\n
We can pass as many arguments to __init__ as we like. All we have to do is to give our arguments names.<\/p>\n
So let\u2019s add arguments named n and r to initialize our attributes self.name and self.roll respectively.<\/p>\n
class Student:\r\n    def __init__(self, n, r):\r\n        self.name = n\r\n        self.roll = r\r\n    def show(self):\r\n        print(\"Name: \", self.name)\r\n        print(\"Roll no.: \", self.roll)\r\n<\/pre>\n
Now that we have implemented<\/strong> a python constructor<\/strong> for our class Student<\/strong>, all we need to do is to pass actual values<\/strong> to the arguments<\/strong> n<\/strong> and r<\/strong>.<\/p>\n
We do this while object instantiation<\/strong>, i.e., creating<\/strong> a new object<\/strong> for the class<\/strong>.<\/p>\n
Let\u2019s create an object s1 from the class Student.<\/p>\n
s1 = Student(\u201cRoss\u201d, 10)<\/pre>\n
We pass two<\/strong> arguments, \u201cRoss\u201d<\/strong> which goes into s1.name<\/strong> and 10<\/strong> which goes into s1.roll<\/strong> for the object s1<\/strong>. We can now call<\/strong> the show()<\/strong> method on the object s1.<\/strong><\/p>\n
>>> s1 = Student(\u201cRoss\u201d, 10)\r\n>>> s1.show()\r\nName: Ross\r\nRoll no.: 10\r\n>>> s2 = Student(\u201cRachel\u201d, 9)\r\n>>> s2.show()\r\nName: Rachel\r\nRoll no.: 9\r\n>>>\r\n<\/pre>\n
We can create<\/strong> as many objects<\/strong> as we want and each object can have different values<\/strong> for their attributes<\/strong>.<\/p>\n
We can also access<\/strong> the attributes<\/strong> of the objects<\/strong> using the dot operator<\/strong> as s1.name<\/strong>, which returns<\/strong> the value<\/strong> of the attribute name of object s1<\/strong>.<\/p>\n
>>> s1.name\r\n\u2018Ross\u2019\r\n>>> s2.roll\r\n9\r\n<\/pre>\n
Types of Python Constructors<\/h3>\n
\"Python<\/a><\/p>\n
Constructors in Python can be of two types<\/strong>:<\/p>\n
a. Parameterized Constructor in Python<\/strong><\/p>\n
Parameterized constructors<\/strong> are ones which have parameters(other than self)<\/strong> defined in the __init__<\/strong> method\u2019s parameter list<\/strong>. This type of constructor can take arguments<\/strong> from the user.<\/p>\n
The Student class we discussed above has a parameterized constructor<\/strong>.<\/p>\n
Student\u2019s constructor takes two arguments (other than self)<\/strong>, n<\/strong> and r<\/strong>.<\/p>\n
b. Non-parameterized Constructor in Python<\/strong><\/p>\n
It is also known as the default constructor<\/strong>.<\/p>\n
The __init__<\/strong> method includes a single parameter<\/strong> self. No other parameters are present in __init__\u2019s parameter list<\/strong>.<\/p>\n
Consequently, this constructor takes no arguments<\/strong> while creating<\/strong> a new object<\/strong>.<\/p>\n
Non-parameterized<\/strong> constructors<\/strong> assign default values to the attributes<\/strong> of the class<\/strong>.<\/p>\n
The student class\u2019s __init__<\/strong> method can be converted to a non-parameterized<\/strong> constructor as follows:<\/p>\n
class Student:\r\n    def __init__(self):\r\n        self.name = \u201cMonica\u201d\r\n        self.roll = 15\r\n    def show(self):\r\n        print(\"Name: \", self.name)\r\n        print(\"Roll no.: \", self.roll)<\/pre>\n
As you would have noticed, the __init__ method<\/strong> has no parameters<\/strong> other than self<\/strong>.<\/p>\n
And the python interpreter<\/strong> assigns the default <\/strong>values \u201cMonica\u201d<\/strong> and 15<\/strong> to the attributes self.name<\/strong> and self.roll<\/strong>, respectively.<\/p>\n
We can create an object s3 as:<\/p>\n
>>> s3 = Student()\r\n>>> s3.show()<\/pre>\n
Output:<\/strong><\/p>\n
Name: Monica
\nRoll no.: 15<\/div>\n
Let me show you a Python trick:<\/strong><\/p>\n
You can combine<\/strong> the concept<\/strong> of default<\/strong> and parameterized constructors<\/strong> in python. <\/strong>We know that methods are functions<\/strong> in disguise<\/strong>.<\/p>\n
Like functions, methods can also take default parameters.<\/strong><\/p>\n
So we can modify<\/strong> the __init__ method<\/strong> of class Student<\/strong> to include default values<\/strong> for its parameters n<\/strong> and r<\/strong>.<\/p>\n
def __init__(self, n = \u201cMonica\u201d , r = 15):\r\n    self.name = n\r\n    self.roll = r<\/pre>\n
Now we pass no argument<\/strong>, name<\/strong> and roll<\/strong> get the default values<\/strong> \u201cMonica\u201d<\/strong> and 15<\/strong>, respectively.<\/p>\n
Else, the attributes<\/strong> will take the values<\/strong> of the arguments<\/strong> we pass<\/strong>.<\/p>\n
Go ahead and test it in Python shell:<\/p>\n
>>> s4 = Student()\r\n>>> s5 = Student(\u201cJoey\u201d, 20)\r\n>>> s4.show()\r\nName: Monica \r\nRoll no.: 15\r\n>>> s5.show()\r\nName: Joey\r\nRoll no.: 20<\/pre>\n
Wrapping Up!<\/h3>\n
This python article introduced you to Python constructors<\/strong>. You also learned why our program needs these constructors<\/strong> in python and how we can implement<\/strong> them.<\/p>\n
We also came across the types<\/strong> of python constructors<\/strong>, ending<\/strong> with a subtle trick<\/strong> you can use in your program while working with constructors<\/strong>.<\/p>\n
Hope you enjoyed the article as it cleared your concepts.<\/p>\n","protected":false},"excerpt":{"rendered":"
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