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Operator Overloading in Python

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parelem

2 Apr, 2009 - 03:25 PM

Post #1

Built in data structures are nice, but have you ever needed or wanted to use something that isn't built in? Have you ever wanted to use built in operators on your data structures? Operator overloading allows you to do so.

In Python the following are operators:

CODE

+       -       *       **      /       //      %
<<      >>      &       |       ^       ~
<       >       <=      >=      ==      !=      <>

Simple examples of a few:

CODE

2 + 1
2*1
2.0-1.0
2/3+1/4

Hey wait a minute what was that last one? 2/3+1/4, according to Python, this is 0! That's not what we wanted now was it? We would like to work with rational numbers in python, so we'll begin by creating a new class called "Rational"

CODE

class Rational(object):
    def __init__(self, numerator=0, denomiator=1):
        self.numer = numerator
        self.denom = denomiator
        self.fixSign()     #will explain later
        self.reduce()     #will explain later

Here I have defined the class and put in an initializing function, what __init__ does is creates an object with the initial parameters if no parameters are supplied for the object. If parameters are supplied, the object will be created with the supplied parameters.

Next we will want to overload some operators so we can use our new class.

CODE

    def __add__(self,rhs):  #self and rhs are Rational objects
        n1 = self.numer    #numerator of the self object
        d1 = self.denom    #denominator of the self object
        n2 = rhs.numer     #numerator of the rhs object
        d2 rhs.denom       #denominator of the rhs object
        r = Rational(d2*n1+d1*n2,d1*d2)    #creates a new object with d2*n1+d1*n2 as the numerator, and d1*d2 as the denominator
        return r
    def __sub__(self,rhs):
        n1 = self.numer    #numerator of the self object
        d1 = self.denom    #denominator of the self object
        n2 = rhs.numer     #numerator of the rhs object
        d2 = rhs.denom       #denominator of the rhs object
        r = Rational(d2*n1-d1*n2,d1*d2)    #creates a new object with d2*n1-d1*n2 as the numerator, and d1*d2 as the denominator
        return r

__add__ and __sub__ are the Python names used for the + and - operators. Now that we have these operators overloaded, we can use + and - on Rational objects just as we would on integers or floats.

Before we get to using our operators, let's first create a few more functions that make life easier.

CODE

    def gcd(self,a,b):
        if b == 0:
            return a
        return self.gcd(b,a%b)
    def fixSigns(self):    #takes - sign out of the denominator
        if self.denom <0:    #check if denominator is negative
            self.numer = -self.numer    #switch sign of numerator
            self.denom = -self.denom    #switch sign of denominator
    def reduce(self):    #reduce the fraction
        d = 1
        if self.denom != 0 and self.numer !=0:      #make sure the fraction != 0
            d = self.gcd(self.numer, self.denom)    #finds the greatest common denominator in order to reduce
        if d > 1:        #reduce away!
            self.numer /= d
            self.denom /= d
    def __str__(self):            #rules for printing our objects
        s = "%d /% d" % (self.numer,self.denom)
        return s

gcd finds the greatest common denominator of the numerator and denominator
fixSigns is used to remove negatives from the denominator
reduce reduces the fraction by using the greatest common denominator function
__str__ gives a rule on how we want our object printed

Complete class:

CODE

class Rational(object):
    def __init__(self, numerator=0, demoniator=1):
        self.numer = numerator
        self.denom = demoniator
        self.fixSign()
        self.reduce()
    def __add__(self,rhs):  #self and rhs are Rational objects
        n1 = self.numer    #numerator of the self object
        d1 = self.denom    #denominator of the self object
        n2 = rhs.numer     #numerator of the rhs object
        d2 rhs.denom       #denominator of the rhs object
        r = Rational(d2*n1+d1*n2,d1*d2)    #creates a new object with d2*n1+d1*n2 as the numerator, and d1*d2 as the denominator
        return r
    def __sub__(self,rhs):
        n1 = self.numer    #numerator of the self object
        d1 = self.denom    #denominator of the self object
        n2 = rhs.numer     #numerator of the rhs object
        d2 = rhs.denom       #denominator of the rhs object
        r = Rational(d2*n1-d1*n2,d1*d2)    #creates a new object with d2*n1-d1*n2 as the numerator, and d1*d2 as the denominator
        return r
    def gcd(self,a,b):
        if b == 0:
            return a
        return self.gcd(b,a%b)
    def fixSigns(self):    #takes - sign out of the denominator
        if self.denom <0:    #check if denominator is negative
            self.numer = -self.numer    #switch sign of numerator
            self.denom = -self.denom    #switch sign of denominator
    def reduce(self):    #reduce the fraction
        d = 1
        if self.denom != 0 and self.numer !=0:      #make sure the fraction != 0
            d = self.gcd(self.numer, self.denom)    #finds the greatest common denominator in order to reduce
        if d > 1:        #reduce away!
            self.numer /= d
            self.denom /= d
    def __str__(self):            #rules for printing our objects
        s = "%d /% d" % (self.numer,self.denom)
        return s

Now that we have all this done, we can start using our class and operators. Let's test!

CODE

from rational import *
def main():
    r1 = Rational(4,9)
    r2 = Rational(2,7)
    r3 = Rational(1,3)
    r4 = Rational(4,6)

    #use our __str__ function
    print "r1: ", r1
    print "r2: ", r2
    print "r3: ", r3
    print "r4: ", r4

    #use our operators

    print "r2-r1: ", (r2-r1)
    print "r1+r3: ", (r1+r3)

    r5 = r3+r4
    print r3, " + ", r2, " = ", r3

    a,b,c,d = 1,4,3,6
    print "%d/%d + %d/%d = %s" %(a,b,c,d,Rational(a,b) + Rational(c,d))
    print "%d/%d - %d/%d = %s" %(a,b,c,d,Rational(a,b) - Rational(c,d))

if __name__ == "__main__":
    main()

Python output:

CODE

r1:  4 / 9
r2:  2 / 7
r3:  1 / 3
r4:  2 / 3
r2-r1:  -10 / 63
r1+r3:  7 / 9
1 / 3  +  2 / 7  =  1 / 3
1/4 + 3/6 = 3 / 4
1/4 - 3/6 = -1 / 4

Now that we've overloaded the + and - operators, we can easily continue to overload more as needed. Perhaps you'd like to overload == (__eq__) or != (__ne__), you can do so by the same method.