How to correctly perform arithmetic on classes instances? In the below example the class represents a House and contains an n_people attribute representing the number of people in that house.
The objective is to perform a sum on the House class instances and return the total number of people in the houses. This seems to work when the House class instances are added together:
class House:
def __init__(self, n_people):
self.n_people = n_people
def __add__(self, n_people):
return n_people + self.n_people
House(3) + House(4)
And fails when an array of House class instances is being summed:
houses = [House(i) for i in range(10)]
sum(houses)
The resulting error is:
TypeError Traceback (most recent call last)
<ipython-input-66-197f26932b89> in <module>
1 houses = [House(i) for i in range(10)]
----> 2 sum(houses)
TypeError: unsupported operand type(s) for +: 'int' and 'House'
What is the correct way to perform arithmetic on class instances in Python?
The problem you're facing is that sum works left-to-right, so after the first two House instances are summed, your code tries to perform int + House, when only House + int is defined.
To solve this, you need to define __radd__, which gets called in that situation:
class House:
def __init__(self, n_people):
self.n_people = n_people
def __add__(self, n_people):
return n_people + self.n_people
def __radd__(self, n_people):
return n_people + self.n_people
House(3) + House(4)
houses = [House(i) for i in range(10)]
sum(houses)
Output:
45