Classes and Objects¶

Creating Classes¶

Syntax:

class ClassName:
   'Optional class documentation string'
   class_suite

Example:

class Student:
   'Common base class for all students'
   studCount = 0

   def __init__(self, name, marks):
      self.name = name
      self.marks = marks
      Student.studCount += 1

   def displayCount(self):
     print ("Total Student %d" % Student.studCount)

   def displayStudent(self):
      print ("Name : ", self.name,  ", Marks: ", self.marks)

Creating Instances¶

This would create first object of Student class
stud1 = Student("Saurabh", 75)
This would create second object of Student class
stud2 = Student("SJ", 90)

Accessing Attributes¶

stud1.displayStudent()
stud2.displayStudent()
print ("Total Student %d" % Student.studCount)

Complete Program

class Student:
   'Common base class for all students'
   studCount = 0

   def __init__(self, name, marks):
      self.name = name
      self.marks = marks
      Student.studCount += 1

   def displayCount(self):
     print ("Total Student %d" % Student.studCount)

   def displayStudent(self):
      print ("Name : ", self.name,  ", Marks: ", self.marks)

#This would create first object of Student class
stud1 = Student("Saurabh", 75)
#This would create second object of Student class
stud2 = Student("SJ", 90)
stud1.displayStudent()
stud2.displayStudent()
print ("Total Student %d" % Student.studCount)

Output:

Name :  Saurabh , Marks:  75
Name :  SJ , Marks:  90
Total Student 2

Built-In Class Attributes¶

Note

__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__ − empty tuple containing the base classes.

class Student:
   'Common base class for all students'
   studCount = 0

   def __init__(self, name, marks):
      self.name = name
      self.marks = marks
      Student.studCount += 1

   def displayCount(self):
     print ("Total Student %d" % Student.studCount)

   def displayStudent(self):
      print ("Name : ", self.name,  ", Marks: ", self.marks)

#This would create first object of Student class
stud1 = Student("Saurabh", 75)
#This would create second object of Student class
stud2 = Student("SJ", 90)
stud1.displayStudent()
stud2.displayStudent()
print ("Total Student %d" % Student.studCount)
print ("Student.__doc__:", Student.__doc__)
print ("Student.__name__:", Student.__name__)
print ("Student.__module__:", Student.__module__)
print ("Student.__bases__:", Student.__bases__)
print ("Student.__dict__:", Student.__dict__ )

Output:

Student.__doc__: Common base class for all students
Student.__name__: Student
Student.__module__: __main__
Student.__bases__: (<class 'object'>,)
Student.__dict__: {'__module__': '__main__', '__doc__': 'Common base class for all students', 'studCount': 2, '__init__': <function Student.__init__ at 0x015993D0>, 'displayCount': <function Student.displayCount at 0x01599388>, 'displayStudent': <function Student.displayStudent at 0x01599340>, '__dict__': <attribute '__dict__' of 'Student' objects>, '__weakref__': <attribute '__weakref__' of 'Student' objects>}

Class Inheritance¶

Syntax:

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

Example:

#!/usr/bin/python3

class Parent:        # define parent class
   parentAttr = 10
   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)

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

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

c = Child()          # instance of child
c.childMethod()      # child calls its method
c.parentMethod()     # calls parent's method
c.setAttr(20)       # again call parent's method
c.getAttr()          # again call parent's method

Output:

Calling child constructor
Calling child method
Calling parent method
Parent attribute : 20

Multiple Inheritance:¶

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

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

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

issubclass(sub, sup)
isinstance(obj, Class)

Overriding Methods¶

#!/usr/bin/python3

class Parent:        # define parent class
   def myMethod(self):
      print ('Calling parent class method')

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

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

Output:

Calling child class method

Magic Methods¶

(Built_in Functions or Base Overloading Methods)

Note

__init__ ( self [,args…] )
__del__( self )
__repr__( self )
__str__( self )
__cmp__ ( self, x )

Overloading Operators¶

#!/usr/bin/python3

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)

Output:

Vector(7,8)