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Comment by gizmo | Hacker News Reader

You can't actually override operator_plus in Python for numbers. Numbers in Python are magic for performance reasons. You can look at the Python bytecode to easily see this:

    def add_five(a):
        return a + 5
    dis.dis(add_five)

And this returns something like:

    LOAD FAST    a
    LOAD CONST   5
    BINARY ADD
    RETURN ACC

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You're right that you can't override __add__ for numbers, but the bytecode doesn't say this. The bytecode isn't specialized to numbers. It will call the __add__ operation on whatever object you give it. The interpreter will probably have a fast path in its implementation of the BINARY_OP instruction that checks for numbers. But that doesn't mean that the BINARY_OP instruction can only handle numbers.

Example:

    >>> class foo(object):
    ...     def __add__(x, y):
    ...         print(f'"adding" {x} and {y} by returning {y}+1', x, y, y)
    ...         return y + 1
    ... 
    >>> f = foo()
    >>> f + 3
    "adding" <__main__.foo object at 0x725f470c4e90> and 3 by returning 3+1 <__main__.foo object at 0x725f470c4e90> 3 3
    4
    >>> add_five(f)
    "adding" <__main__.foo object at 0x725f470c4e90> and 5 by returning 5+1 <__main__.foo object at 0x725f470c4e90> 5 5
    6

(Not sure why there's extra garbage printed, my Python is a bit rusty.)

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