Python Class

Python is a completely object-oriented language. You have been working with classes and objects right from the beginning of these tutorials. Every element in a Python program is an object of a class. A number, string, list, dictionary, etc., used in a program is an object of a corresponding built-in class. You can retrieve the class name of variables or objects using the type() method, as shown below.

>>> num=20
>>> type(num)
<class 'int'>            
>>> s="Python"
>>> type(s) 
<class 'str'>

Defining a Class

A class in Python can be defined using the class keyword.

class <ClassName>:
    <statement1>
    <statement2>
    .
    .
    <statementN>

As per the syntax above, a class is defined using the class keyword followed by the class name and : operator after the class name, which allows you to continue in the next indented line to define class members. The followings are class members.

1. Class Attributes
2. Constructor
3. Instance Attributes
4. Properties
5. Class Methods

A class can also be defined without any members. The following example defines an empty class using the pass keyword.

class Student:
    pass

Class instantiation uses function notation. To create an object of the class, just call a class like a parameterless function that returns a new object of the class, as shown below.

std = Student()

Above, Student() returns an object of the Student class, which is assigned to a local variable std. The Student class is an empty class because it does not contain any members.

Class Attributes

Class attributes are the variables defined directly in the class that are shared by all objects of the class. Class attributes can be accessed using the class name as well as using the objects.

class Student:
    schoolName = 'XYZ School' 

Above, the schoolName is a class attribute defined inside a class. The value of the schoolName will remain the same for all the objects unless modified explicitly.

>>> Student.schoolName
'XYZ School' 
>>> std = Student()
>>> std.schoolName
'XYZ School' 

As you can see, a class attribute is accessed by Student.schoolName as well as std.schoolName. Changing the value of class attribute using the class name would change it across all instances. However, changing class attribute value using instance will not reflect to other instances or class.

>>> Student.schoolName = 'ABC School' # change attribute value using class name
>>> std = Student()
>>> std.schoolName
'ABC School'   # value changed for all instances
>>> std.schoolName = 'My School'  # changing instance's attribute
>>> std.schoolName
'My School' 
>>> Student.schoolName # instance level change not reflectd to class attribute
'ABC School' 
>>> std2 = Student()
>>> std2.schoolName
'ABC School' 

The following example demonstrates the use of class attribute count.

class Student:
    count = 0
    def __init__(self):
        Student.count += 1                

In the above example, count is an attribute in the Student class. Whenever a new object is created, the value of count is incremented by 1. You can now access the count attribute after creating the objects, as shown below.

>>> std1=Student()
>>> Student.count
1
>>> std2 = Student()
>>> Student.count
2

Constructor

In Python, the constructor method is invoked automatically whenever a new object of a class is instantiated, same as constructors in C# or Java. The constructor must have a special name __init__() and a special parameter called self.

The following example defines a constructor.

class Student:
    def __init__(self): # constructor method
        print('Constructor invoked')

Now, whenever you create an object of the Student class, the __init__() constructor method will be called, as shown below.

>>>s1 = Student()
Constructor invoked
>>>s2 = Student()
Constructor invoked

The constructor in Python is used to define the attributes of an instance and assign values to them.

Instance Attributes

Instance attributes are attributes or properties attached to an instance of a class. Instance attributes are defined in the constructor.

The following example defines instance attributes name and age in the constructor.

class Student:
    schoolName = 'XYZ School' # class attribute

    def __init__(self): # constructor
        self.name = '' # instance attribute
        self.age = 0 # instance attribute

An instance attribute can be accessed using dot notation: [instance name].[attribute name], as shown below.

>>> std = Student()
>>> std.name
''
>>> std.age
0

You can set the value of attributes using the dot notation, as shown below.

>>> std = Student()
>>> std.name = "Bill" # assign value to instance attribute
>>> std.age=25        # assign value to instance attribute
>>> std.name          # access instance attribute value
Bill
>>> std.age           # access value to instance attribute
25

You can specify the values of instance attributes through the constructor. The following constructor includes the name and age parameters, other than the self parameter.

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

Now, you can specify the values while creating an instance, as shown below.

>>> std = Student('Bill',25)
>>> std.name
'Bill'
>>> std.age
25

You can also set default values to the instance attributes. The following code sets the default values of the constructor parameters. So, if the values are not provided when creating an object, the values will be assigned latter.

class Student:
    def __init__(self, name="Guest", age=25)
        self.name=name
        self.age=age

Now, you can create an object with default values, as shown below.

>>> std = Student()
>>> std.name
'Guest'
>>> std.age
25

Visit class attributes vs instance attributes in Python for more information.

Class Properties

In Python, a property in the class can be defined using the property() function.

The property() method in Python provides an interface to instance attributes. It encapsulates instance attributes and provides a property, same as Java and C#.

The property() method takes the get, set and delete methods as arguments and returns an object of the property class.

The following example demonstrates how to create a property in Python using the property() function.

class Student:
    def __init__(self):
        self.__name=''
    def setname(self, name):
        print('setname() called')
        self.__name=name
    def getname(self):
        print('getname() called')
        return self.__name
    name=property(getname, setname)

In the above example, property(getname, setname) returns the property object and assigns it to name. Thus, the name property hides the private instance attribute __name. The name property is accessed directly, but internally it will invoke the getname() or setname() method, as shown below.

>>> std = Student()
>>> std.name="Steve"
setname() called
>>> std.name
getname() called
'Steve'

It is recommended to use the property decorator instead of the property() method.

Class Methods

You can define as many methods as you want in a class using the def keyword. Each method must have the first parameter, generally named as self, which refers to the calling instance.

class Student:
    def displayInfo(self): # class method
        print('Student Information')

Self is just a conventional name for the first argument of a method in the class. A method defined as mymethod(self, a, b) should be called as x.mymethod(a, b) for the object x of the class.

The above class method can be called as a normal function, as shown below.

>>> std = Student()
>>> std.displayInfo()
'Student Information'

The first parameter of the method need not be named self. You can give any name that refers to the instance of the calling method. The following displayInfo() method names the first parameter as obj instead of self and that works perfectly fine.

class Student:
    def displayInfo(obj): # class method
        print('Student Information')

Defining a method in the class without the self parameter would raise an exception when calling a method.

class Student:
    def displayInfo(): # method without self parameter
        print('Student Information')

>>> std = Student()
>>> std.displayInfo()
Traceback (most recent call last):
std.displayInfo()
TypeError: displayInfo() takes 0 positional arguments but 1 was given

The method can access instance attributes using the self parameter.

class Student:
    def __init__(self, name, age): 
        self.name = name 
        self.age = age 
    def displayInfo(self): # class method
        print('Student Name: ', self.name,', Age: ', self.age)

You can now invoke the method, as shown below.

>>> std = Student('Steve', 25)
>>> std.displayInfo()
Student Name: Steve , Age: 25 

Deleting Attribute, Object, Class

You can delete attributes, objects, or the class itself, using the del keyword, as shown below.

>>> std = Student('Steve', 25)
>>> del std.name # deleting attribute
>>> std.name
Traceback (most recent call last):
File "<pyshell#42>", line 1, in <module>
std.name
AttributeError: 'Student' object has no attribute 'name'
>>> del std  # deleting object
>>> std.name  
Traceback (most recent call last):
File "<pyshell#42>", line 1, in <module>
std.name
NameError: name 'std' is not defined
>>> del Student  # deleting class
>>> std = Student('Steve', 25)
Traceback (most recent call last):
File "<pyshell#42>", line 1, in <module>
std = Student()
NameError: name 'Student' is not defined