Python - Object and Classes

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Python is an object-oriented programming language, which means that each and every element used within a Python program is an object of one or another class. For instance, numbers, strings, lists, dictionaries, and other similar entities of a program are objects of the corresponding built-in class.

In Python, the Object class is the base or parent class for all the classes, built-in as well as user defined.

Example

If we want to see which attribute belongs to which built-in class, we can use the Python type() function as demonstrated in the below example −

num = 20
print (type(num))
num1 = 55.50
print (type(num1))
s = "TutorialsPoint"
print (type(s))
dct = {'a':1,'b':2,'c':3}
print (type(dct))
def SayHello():
   print ("Hello World")
   return
print (type(SayHello))

When you execute this code, it will produce the following output −

<class 'int'>
<class 'float'>
<class 'str'>
<class 'dict'>
<class 'function'>

Creating Classes in Python

The class keyword is used to define a new class. The name of the class immediately follows the keyword class followed by a colon as follows −

class ClassName:
   'Optional class documentation string'
   class_suite

• The class has a documentation string, which can be accessed via ClassName.__doc__.

• The class_suite consists of all the component statements defining class members, data attributes and functions.

Example

Following is the example of a simple Python class −

class Employee:
   'Common base class for all employees'
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print "Total Employee %d" % Employee.empCount

   def displayEmployee(self):
      print "Name : ", self.name,  ", Salary: ", self.salary

• The variable empCount is a class variable whose value is shared among all instances of a this class. This can be accessed as Employee.empCount from inside the class or outside the class.

• The first method __init__() is a special method, which is called class constructor or initialization method that Python calls when you create a new instance of this class.

• You declare other class methods like normal functions with the exception that the first argument to each method is self. Python adds the self argument to the list for you; you do not need to include it when you call the methods.

Creating Objects of Classes in Python

To create instances of a class, you call the class using class name and pass in whatever arguments its __init__ method accepts.

# This would create first object of Employee class
emp1 = Employee("Zara", 2000)
# This would create second object of Employee class
emp2 = Employee("Manni", 5000)

Accessing Attributes of Objects in Python

You access the object's attributes using the dot operator with object. Class variable would be accessed using class name as follows −

emp1.displayEmployee()
emp2.displayEmployee()
print ("Total Employee %d" % Employee.empCount)

Now, putting all the concepts together −

class Employee:
   "Common base class for all employees"
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print ("Total Employee %d" % Employee.empCount)

   def displayEmployee(self):
      print ("Name : ", self.name,  ", Salary: ", self.salary)

# This would create first object of Employee class
emp1 = Employee("Zara", 2000)
# This would create second object of Employee class
emp2 = Employee("Manni", 5000)
emp1.displayEmployee()
emp2.displayEmployee()
print ("Total Employee %d" % Employee.empCount)

When the above code is executed, it produces the following result −

Name :  Zara , Salary:  2000
Name :  Manni , Salary:  5000
Total Employee 2

You can add, remove, or modify attributes of classes and objects at any time −

# Add an 'age' attribute
emp1.age = 7  
# Modify 'age' attribute
emp1.age = 8  
# Delete 'age' attribute
del emp1.age  

Instead of using the normal statements to access attributes, you can also use the following functions −

• The getattr(obj, name[, default]) − to access the attribute of object.

• The hasattr(obj,name) − to check if an attribute exists or not.

• The setattr(obj,name,value) − to set an attribute. If attribute does not exist, then it would be created.

• The delattr(obj, name) − to delete an attribute.

# Returns true if 'age' attribute exists
hasattr(emp1, 'age')   
# Returns value of 'age' attribute
getattr(emp1, 'age')    
# Set attribute 'age' at 8
setattr(emp1, 'age', 8) 
# Delete attribute 'age'
delattr(empl, 'age')    

Built-In Class Attributes in Python

Every Python class keeps following built-in attributes and they can be accessed using dot operator like any other attribute −

• __dict__ − Dictionary containing the class's namespace.

• __doc__ − Class documentation string or none, if undefined.

• __name__ − Class name.

• __module__ − Module name in which the class is defined. This attribute is "__main__" in interactive mode.

• __bases__ − A possibly empty tuple containing the base classes, in the order of their occurrence in the base class list.

For the above class let us try to access all these attributes −

class Employee:
   'Common base class for all employees'
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print ("Total Employee %d" % Employee.empCount)

   def displayEmployee(self):
      print ("Name : ", self.name,  ", Salary: ", self.salary)

print ("Employee.__doc__:", Employee.__doc__)
print ("Employee.__name__:", Employee.__name__)
print ("Employee.__module__:", Employee.__module__)
print ("Employee.__bases__:", Employee.__bases__)
print ("Employee.__dict__:", Employee.__dict__)

When the above code is executed, it produces the following result −

Employee.__doc__: Common base class for all employees
Employee.__name__: Employee
Employee.__module__: __main__
Employee.__bases__: ()
Employee.__dict__: {'__module__': '__main__', 'displayCount':
<function displayCount at 0xb7c84994>, 'empCount': 2, 
'displayEmployee': <function displayEmployee at 0xb7c8441c>, 
'__doc__': 'Common base class for all employees', 
'__init__': <function __init__ at 0xb7c846bc>}

Destroying Objects (Garbage Collection) in Python

Python deletes unneeded objects (built-in types or class instances) automatically to free the memory space. The process by which Python periodically reclaims blocks of memory that no longer are in use is termed Garbage Collection.

Python's garbage collector runs during program execution and is triggered when an object's reference count reaches zero. An object's reference count changes as the number of aliases that point to it changes.

An object's reference count increases when it is assigned a new name or placed in a container (list, tuple, or dictionary). The object's reference count decreases when it's deleted with del, its reference is reassigned, or its reference goes out of scope. When an object's reference count reaches zero, Python collects it automatically.

# Create object <40>
a = 40      
# Increase ref. count  of <40> 
b = a       
# Increase ref. count  of <40> 
c = [b]     

# Decrease ref. count  of <40>
del a       
# Decrease ref. count  of <40>
b = 100      
# Decrease ref. count  of <40>
c[0] = -1    

You normally will not notice when the garbage collector destroys an orphaned instance and reclaims its space. But a class can implement the special method __del__(), called a destructor, that is invoked when the instance is about to be destroyed. This method might be used to clean up any non memory resources used by an instance.

Example

The __del__() destructor prints the class name of an instance that is about to be destroyed as shown in the below code block −

class Point:
   def __init__( self, x=0, y=0):
      self.x = x
      self.y = y
   def __del__(self):
      class_name = self.__class__.__name__
      print (class_name, "destroyed")

pt1 = Point()
pt2 = pt1
pt3 = pt1
# prints the ids of the obejcts
print (id(pt1), id(pt2), id(pt3))
del pt1
del pt2
del pt3

On executing, the above code will produces following result −

135007479444176 135007479444176 135007479444176
Point destroyed

Class Inheritance in Python

Instead of starting from scratch, you can create a class by deriving it from a preexisting class by listing the parent class in parentheses after the new class name.

The child class inherits the attributes of its parent class, and you can use those attributes as if they were defined in the child class. A child class can also override data members and methods from the parent.

Syntax

Derived classes are declared much like their parent class; however, a list of base classes to inherit from is given after the class name −

class SubClassName (ParentClass1[, ParentClass2, ...]):
   'Optional class documentation string'
   class_suite

Example

#!/usr/bin/python
# define parent class
class Parent:        
   parentAttr = 100
   def __init__(self):
      print ("Calling parent constructor")

   def parentMethod(self):
      print ("Calling parent method")

   def setAttr(self, attr):
      Parent.parentAttr = attr

   def getAttr(self):
      print ("Parent attribute :", Parent.parentAttr)

# define child class
class Child(Parent): 
   def __init__(self):
      print ("Calling child constructor")

   def childMethod(self):
      print ("Calling child method")

# instance of child
c = Child()  
# child calls its method        
c.childMethod() 
# calls parent's method     
c.parentMethod()  
# again call parent's method   
c.setAttr(200)  
# again call parent's method     
c.getAttr()          

When the above code is executed, it produces the following result −

Calling child constructor
Calling child method
Calling parent method
Parent attribute : 200

Similar way, you can drive a class from multiple parent classes as follows −

class A:        # define your class A
.....

class B:         # define your class B
.....

class C(A, B):   # subclass of A and B
.....

You can use issubclass() or isinstance() functions to check a relationships of two classes and instances.

• The issubclass(sub, sup) boolean function returns true if the given subclass sub is indeed a subclass of the superclass sup.

• The isinstance(obj, Class) boolean function returns true if obj is an instance of class Class or is an instance of a subclass of Class

Overriding Methods in Python

You can always override your parent class methods. One reason for overriding parent's methods is because you may want special or different functionality in your subclass.

Example

#!/usr/bin/python
# define parent class
class Parent:        
   def myMethod(self):
      print ("Calling parent method")

# define child class
class Child(Parent): 
   def myMethod(self):
      print ("Calling child method")

# instance of child
c = Child()
# child calls overridden method          
c.myMethod()         

When the above code is executed, it produces the following result −

Calling child method

Base Overloading Methods in Python

Following table lists some generic functionality that you can override in your own classes −

Overloading Operators in Python

Suppose you have created a Vector class to represent two-dimensional vectors, what happens when you use the plus operator to add them? Most likely Python will yell at you.

You could, however, define the __add__ method in your class to perform vector addition and then the plus operator would behave as per expectation −

Example

#!/usr/bin/python

class Vector:
   def __init__(self, a, b):
      self.a = a
      self.b = b

   def __str__(self):
      return 'Vector (%d, %d)' % (self.a, self.b)
   
   def __add__(self,other):
      return Vector(self.a + other.a, self.b + other.b)

v1 = Vector(2,10)
v2 = Vector(5,-2)
print (v1 + v2)

When the above code is executed, it produces the following result −

Vector(7,8)

Data Hiding in Python

An object's attributes may or may not be visible outside the class definition. You need to name attributes with a double underscore prefix, and those attributes then are not be directly visible to outsiders.

Example

class JustCounter:
   __secretCount = 0
  
   def count(self):
      self.__secretCount += 1
      print self.__secretCount

counter = JustCounter()
counter.count()
counter.count()
print counter.__secretCount

When the above code is executed, it produces the following result −

1
2
ERROR!
Traceback (most recent call last):
  File <main.py>", line 11, in <module>
AttributeError: 'JustCounter' object has no attribute '__secretCount'

Python protects those members by internally changing the name to include the class name. You can access such attributes as object._className__attrName. If you would replace your last line as following, then it works for you −

print(counter._JustCounter__secretCount)

When the above code is executed, it produces the following result −

1
2
2