Python operator overloading with multiple operands

Question

I know I can do simple operator overloading in python by the following way.

Let say overloading '+' operator.

class A(object):
  def __init__(self,value):
    self.value = value

  def __add__(self,other):
    return self.value + other.value

a1 = A(10)
a2 = A(20)
print a1 + a2

But It fails when I try to do the following,

a1 = A(10)
a2 = A(20)
a3 = A(30)
print a1 + a2 + a3

Since the __add__ accepts only 2 parameters. What is the best solution to achieve operator overloading with n number of operands.

Dimitris Fasarakis Hilliard · Accepted Answer · 2016-09-15 14:31:21Z

4

This is failing because a1 + a2 return an int instance and its __add__ is called which doesn't support addition with the custom class A; you could return a A instance in __add__ to eliminate the Exception for this specific operation:

class A(object):
  def __init__(self,value):
    self.value = value

  def __add__(self,other):
    return type(self)(self.value + other.value)

Adding them together now behaves in the expected way:

>>> a1 = A(10)
>>> a2 = A(20)
>>> a3 = A(30)
>>> print(a1 + a2 + a3)
<__main__.A object at 0x7f2acd7d25c0>
>>> print((a1 + a2 + a3).value)
60

This class of course suffers from the same issues with other operations; you need to implement the other dunders to return an instance of your class or else you'll bump into the same result with other operations.

If you want a nice result displayed when you print these objects, you should also implement __str__ to return the value when called:

class A(object):
    def __init__(self,value):
        self.value = value

    def __add__(self,other):
        return A(self.value + other.value)

    def __str__(self):
        return "{}".format(self.value)

Now printing has the effect you need:

>>> print(a1 + a2 + a3)
60

edited Sep 15, 2016 at 14:31

answered Sep 15, 2016 at 14:10

Dimitris Fasarakis Hilliard

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4 Comments

Lev Levitsky Over a year ago

Why temporarily? Looks like the proper solution to me.

Dimitris Fasarakis Hilliard Over a year ago

temporarily might be a wrong word, point is this fixes the issue for the specific operation but in general you'll get weird results if you don't implement all required operations.

karthi_ms Over a year ago

Thanks.But I wanted to have the result by just giving a1 + a2 + a3 ,not using the value attribute externally.

Dimitris Fasarakis Hilliard Over a year ago

Then you overload __str__ (and maybe __repr__) @karthi_ms to return the result :-)

Pedru · Accepted Answer · 2016-09-15 14:10:21Z

0

The problem is that

a1 + a2 + a3 gives 30 + a3

30 is an int, and ints do not know how to sum to A

You should return an instance of A in your __add__ function

answered Sep 15, 2016 at 14:10

Pedru

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