Defining add (and other magic methods) dynamically

Question

I've heard that a + b expands to a.__add__(b). However, it doesn't work like that in my code. Take this code:

class TestAdd:
    def __init__(self, a):
        self.a = a
    def __getattr__(self, name):
        if name == '__add__':
            return (lambda y: self.a + y)

When I try TestAdd(5).__add__(1), it returns 6 correctly. When I try TestAdd(5) + 1, it raises this error:

TypeError                                 Traceback (most recent call last)
<ipython-input-3-1c04c7082b89> in <module>
----> 1 TestAdd(5) + 1

TypeError: unsupported operand type(s) for +: 'TestAdd' and 'int'

Why is this? Am I missing something?

Why don't you just define `__add__`? What are your constraints which require you to do this instead? — Ted Klein Bergman, Nov 23 '20 at 20:16

score 0 · Answer 1 · answered Nov 23 '20 at 20:10

0

You can't use __getattr__ to dynamically return a magic method; this is documented in depth in 3.3.10 Special method lookup in the manual.

answered Nov 23 '20 at 20:10

AKX

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score 0 · Accepted Answer · answered Nov 23 '20 at 20:11

0

Magic methods aren't looked up on instances, but on classes. TestAdd(5) + 1 is more precisely equivalent to tmp = TestAdd(5); TestAdd.__add__(tmp, 1), and you haven't defined TestAdd.__add__.

answered Nov 23 '20 at 20:11

chepner

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Defining __add__ (and other magic methods) dynamically

2 Answers2

Defining add (and other magic methods) dynamically