Class declarations in python

Python Classes and Objects

Almost everything in Python is an object, with its properties and methods.

A Class is like an object constructor, or a «blueprint» for creating objects.

Create a Class

To create a class, use the keyword class :

Example

Create a class named MyClass, with a property named x:

Create Object

Now we can use the class named MyClass to create objects:

Example

Create an object named p1, and print the value of x:

The __init__() Function

The examples above are classes and objects in their simplest form, and are not really useful in real life applications.

To understand the meaning of classes we have to understand the built-in __init__() function.

All classes have a function called __init__(), which is always executed when the class is being initiated.

Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created:

Example

Create a class named Person, use the __init__() function to assign values for name and age:

class Person:
def __init__(self, name, age):
self.name = name
self.age = age

Note: The __init__() function is called automatically every time the class is being used to create a new object.

The __str__() Function

The __str__() function controls what should be returned when the class object is represented as a string.

If the __str__() function is not set, the string representation of the object is returned:

Example

The string representation of an object WITHOUT the __str__() function:

class Person:
def __init__(self, name, age):
self.name = name
self.age = age

Example

The string representation of an object WITH the __str__() function:

class Person:
def __init__(self, name, age):
self.name = name
self.age = age

Object Methods

Objects can also contain methods. Methods in objects are functions that belong to the object.

Let us create a method in the Person class:

Example

Insert a function that prints a greeting, and execute it on the p1 object:

class Person:
def __init__(self, name, age):
self.name = name
self.age = age

def myfunc(self):
print(«Hello my name is » + self.name)

p1 = Person(«John», 36)
p1.myfunc()

Note: The self parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.

The self Parameter

The self parameter is a reference to the current instance of the class, and is used to access variables that belongs to the class.

It does not have to be named self , you can call it whatever you like, but it has to be the first parameter of any function in the class:

Example

Use the words mysillyobject and abc instead of self:

class Person:
def __init__(mysillyobject, name, age):
mysillyobject.name = name
mysillyobject.age = age

def myfunc(abc):
print(«Hello my name is » + abc.name)

p1 = Person(«John», 36)
p1.myfunc()

Modify Object Properties

You can modify properties on objects like this:

Example

Delete Object Properties

You can delete properties on objects by using the del keyword:

Example

Delete the age property from the p1 object:

Delete Objects

You can delete objects by using the del keyword:

Example

The pass Statement

class definitions cannot be empty, but if you for some reason have a class definition with no content, put in the pass statement to avoid getting an error.

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Python Classes and Objects

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Python is an object-oriented programming language. Python Classes and Objects are the core building blocks of Python programming language.

Python Class

By this time, all of you should have already learned about Python Data Types. If you remember, basic data types in python refer to only one kind of data at a time. How would it be if you could declare a data type which itself contains more than one data types and can work with them with the help of any function? Python class gives you that opportunity. Python class is the blueprint on which instances of the class are created.

Simple Python Class Declaration

class ClassName: # list of python class variables # python class constructor # python class method definitions 

#definition of the class starts here class Person: #initializing the variables name = "" age = 0 #defining constructor def __init__(self, personName, personAge): self.name = personName self.age = personAge #defining class methods def showName(self): print(self.name) def showAge(self): print(self.age) #end of the class definition # Create an object of the class person1 = Person("John", 23) #Create another object of the same class person2 = Person("Anne", 102) #call member methods of the objects person1.showAge() person2.showName() 

This example is pretty much self-explanatory. As we know, the lines starting with “#” are python comments. The comments explain the next executable steps. This code produces the following output.

Python Class Definition

Python Class Variables

 #initializing the variables name = "" age = 0 

‘name’ and ‘age’ are two member variables of the class ‘Person’. Every time we declare an object of this class, it will contain these two variables as its member. This part is optional as they can be initialized by the constructor.

Python Class Constructor

 #defining constructor def __init__(self, personName, personAge): self.name = personName self.age = personAge 

Python class constructor is the first piece of code to be executed when you create a new object of a class. Primarily, the constructor can be used to put values in the member variables. You may also print messages in the constructor to be confirmed whether the object has been created. We shall learn a greater role of constructor once we get to know about python inheritance. The constructor method starts with def __init__. Afterward, the first parameter must be ‘self’, as it passes a reference to the instance of the class itself. You can also add additional parameters like the way it is shown in the example. ‘personName’ and ‘personAge’ are two parameters to sent when a new object is to be created.

Python Class Methods

#defining python class methods def showName(self): print(self.name) 

def method_name(self, parameter 1, parameter 2, …….) statements…….. return value (if required) 

In the pre-stated example, we’ve seen that the method showName() prints value of ‘name’ of that object. We shall discuss a lot more about python methods some other day.

Python Class Object

# Create an object of the class person1 = Person("Richard", 23) #Create another object of the same class person2 = Person("Anne", 30) #call member methods of the objects person1.showAge() person2.showName() 

The way objects are created in python is quite simple. At first, you put the name of the new object which is followed by the assignment operator and the name of the class with parameters (as defined in the constructor). Remember, the number and type of parameters should be compatible with the parameters received in the constructor function. When the object has been created, member methods can be called and member attributes can be accessed (provided they are accessible).

#print the name of person1 by directly accessing the ‘name’ attribute print(person1.name) 

That’s all for the basics of python class. As we are going to learn about python object-oriented features like inheritance, polymorphism in the subsequent tutorials, we shall learn more about python class and its features. Till then, happy coding and good bye! Feel free to comment if you have any query. Reference: Python.org Documentation

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