Python create built-in method for class

Question

I made a class that can do some fraction arithmetic. I change the built-in method of __add__, __sub__,__mul__,and __div__ so that it can do arithmetic with fractions. I can use it with the +, - , * , / symbols. My question is, what do I have to do to be able to use __iadd__ as +=.

class Fraction:
    def __init__(self,num,den):
        self.num = num
        self.den = den
    def __str__(self):
        return str(self.num)+" / "+str(self.den)

    def __add__(self,other):
        num = self.num * other.den + other.num * self.den
        den = self.den * other.den
        common = self.gcf(num,den)
        return Fraction(num/common , den/common)

    def __iadd__(self,other):
        self.num = self.num * other.den + other.num * self.den
        self.den = self.den * other.den
        common = self.gcf(self.num,self.den)
        self.num = self.num/common
        self.den = self.den/common

always subclass from object if you are using Python 2.x.

Ozgur Vatansever
– Ozgur Vatansever

2015-04-11 16:43:20 +00:00
Commented Apr 11, 2015 at 16:43 — Ozgur Vatansever
– Ozgur Vatansever, Commented Apr 11, 2015 at 16:43

user4815162342 · Accepted Answer · 2015-04-11 16:35:16Z

1

You are missing return self at the end of your __iadd__ implementation. Augmented assignment methods are allowed to return different instances, which is why return self is necessary.

In an unrelated note, you can reduce some code duplication by implementing addition in terms of in-place addition, like this:

def __add__(self, other):
    clone = Fraction(self.num, self.den)
    clone += other
    return clone

edited Apr 11, 2015 at 16:35

answered Apr 11, 2015 at 16:32

user4815162342

158k22 gold badges347 silver badges417 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

a wei Over a year ago

Actually after looking at my code I realized that I did that on purpose; if you use += it doesn't return a number.

user4815162342 Over a year ago

@william880513 That was likely the result of the bug explained in the first paragraph of my answer.

a wei Over a year ago

What I am trying to do is so that I can change the value of a fraction by doing fraction1 += fraction2, instead of fraction1.__iadd__(fraction2)

user4815162342 Over a year ago

@william880513 ...and definining __iadd__ will accomplish exactly that — provided you remember to return self at the end of it. The suggestion for how to avoid code duplication in __add__ is just an aside.

a wei Over a year ago

link This explain why return self is necessary

Collectives™ on Stack Overflow

Python create built-in method for class

1 Answer 1

5 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related