How to implement Integer+MyObject()?

Question

Asking just out of curiocity. I've been playing around with python, and noticed this behaviour:

(2).__add__(2) results in 4, which is totally expected;
(2).__add__('a') results in NotImplemented - nothing weird yet;
'a'.__add__(2) produces an exception.

But, when trying to create my own class like so:

class Integer:
    def __init__(self, d):
        self.val = d
    def __add__(self, i):
        return self.val+int(i)
    def __iadd__(self, i):
        self.val += int(i)
        return self
    def __int__(self):
        return self.val

and comparing it to decimal.Decimal, some weird stuff happens. They behave similarly in this case:

import decimal

a = Integer(2)
a += 2
print(int(a+2))

d = decimal.Decimal(2)
d += 2
print(int(d+2))

However, this is not as similar: 2+d works normally while 2+a throws an exception:

Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    2+a
TypeError: unsupported operand type(s) for +: 'int' and 'Integer'

even though (2).__add__(a) and (2).__add__(d) both produce NotImplemented and a.__add__(2) and d.__add__(2) work normally. What exactly is happening with decimal.Decimal and how can I make this happen with my class?

throws an exception can you please paste it here?

TayTay
– TayTay

2020-05-13 13:45:57 +00:00
Commented May 13, 2020 at 13:45 — TayTay
– TayTay, Commented May 13, 2020 at 13:45
@Tgsmith61591 sure

rizerphe
– rizerphe

2020-05-13 13:46:23 +00:00
Commented May 13, 2020 at 13:46 — rizerphe
– rizerphe, Commented May 13, 2020 at 13:46
That's why __radd__ exists

ForceBru
– ForceBru

2020-05-13 13:48:21 +00:00
Commented May 13, 2020 at 13:48 — ForceBru
– ForceBru, Commented May 13, 2020 at 13:48

jkr · Accepted Answer · 2020-05-13 13:52:27Z

2

Implement __radd__ too:

class Integer:
    def __init__(self, d):
        self.val = d

    def __add__(self, i):
        return self.val + int(i)

    def __radd__(self, i):
        return self.__add__(i)  # might be different if operation is not commutative

    def __iadd__(self, i):
        self.val += int(i)
        return self

    def __int__(self):
        return self.val


x = Integer(5) + 5
y = 5 + Integer(5)

edited May 13, 2020 at 13:52

jkr

19.6k5 gold badges48 silver badges78 bronze badges

answered May 13, 2020 at 13:49

AKX

171k16 gold badges147 silver badges229 bronze badges

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

4 Comments

rizerphe Over a year ago

How exactly does that work? Is __radd__ called only after __add__ returns NotImplemented? Are there any special cases ( __radd__ returns NotImplemented or something) and, if there are, what would happen in those?

rizerphe Over a year ago

Why isn't __radd__ documented inside operator module?

AKX Over a year ago

Re your first question: yes, it looks like reverse functions are called if the forward functions raise NotImplemented: github.com/python/cpython/blob/…

AKX Over a year ago

Re your second question: because it doesn't need to have a mapping for an operator function.

Collectives™ on Stack Overflow

How to implement Integer+MyObject()?

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related