Python Operator Overloading

Education is not limited to just classrooms. It can be gained anytime, anywhere... - Ravi Ranjan (M.Tech-NIT)

You have already seen you can use +  operator for adding numbers and at the same time to concatenate strings. It is possible because +  operator is overloaded by both int  class andstr  class. The operators are actually methods defined in respective classes. Defining methods for operators is known as operator overloading. For e.g. To use +  operator with custom objects  you need to define a method called __add__  .

Let’s take an example to understand better

 

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import math

 

class Circle:

 

    def __init__(self, radius):

        self.__radius = radius

 

    def setRadius(self, radius):

        self.__radius = radius

 

    def getRadius(self):

        return self.__radius

 

    def area(self):

        return math.pi * self.__radius ** 2

 

    def __add__(self, another_circle):

        return Circle( self.__radius + another_circle.__radius )

 

c1 = Circle(4)

print(c1.getRadius())

 

c2 = Circle(5)

print(c2.getRadius())

 

c3 = c1 + c2 # This became possible because we have overloaded + operator by adding a    method named __add__

print(c3.getRadius())

Expected Output:

 

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9

In the above example we have added __add__  method which allows use to use +  operator to add two circle objects. Inside the __add__  method we are creating a new object and returning it to the caller.

python has many other special methods like __add__ , see the list below.

OPERATOR FUNCTION   METHOD DESCRIPTION
  __add__(self, other)   Addition
  __mul__(self, other)   Multiplication
  __sub__(self, other)   Subtraction
  __mod__(self, other)   Remainder
  __truediv__(self, other)   Division
  __lt__(self, other)   Less than
<=    __le__(self, other)   Less than or equal to
==    __eq__(self, other)   Equal to
!=    __ne__(self, other)   Not equal to
  __gt__(self, other)  Greater than
>=    __ge__(self, other)   Greater than or equal to
[index]    __getitem__(self, index)   Index operator
in    __contains__(self, value)  Check membership
len  __len__(self)   The number of elements
str  __str__(self)   The string representation

Program below is using some of the above mentioned functions to overload operators.

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import math

 

class Circle:

 

    def __init__(self, radius):

        self.__radius = radius

 

    def setRadius(self, radius):

        self.__radius = radius

 

    def getRadius(self):

        return self.__radius

 

    def area(self):

        return math.pi * self.__radius ** 2

 

    def __add__(self, another_circle):

        return Circle( self.__radius + another_circle.__radius )

 

    def __gt__(self, another_circle):

        return self.__radius > another_circle.__radius

 

    def __lt__(self, another_circle):

        return self.__radius < another_circle.__radius

 

    def __str__(self):

        return "Circle with radius " + str(self.__radius)

 

c1 = Circle(4)

print(c1.getRadius())

 

c2 = Circle(5)

print(c2.getRadius())

 

c3 = c1 + c2

print(c3.getRadius())

 

print( c3 > c2) # Became possible because we have added __gt__ method

 

print( c1 < c2) # Became possible because we have added __lt__ method

 

print(c3) # Became possible because we have added __str__ method

Expected Output:

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True

True

Circle with radius 9

 

Next lesson is inheritance and polymorphism.