Mark Dickinson added the comment:
> could you point me to where this logic is implemented in CPython's source?
Most of the relevant code is in Objects/abstract.c and Objects/typeobject.c.
A BINARY_ADD opcode (for example) ends up calling PyNumber_Add:
https://github.com/python/cpython/blob/v3.6.1/Objects/abstract.c#L913
which in turn calls binary_op1, where you see an explicit check for the nb_add
slots of the two operands being identical:
https://github.com/python/cpython/blob/v3.6.1/Objects/abstract.c#L769-L770
For a user-defined class, the slots themselves are defined in typeobject.c.
Here's where nb_add is defined:
https://github.com/python/cpython/blob/v3.6.1/Objects/typeobject.c#L5952
and here's the explicit check for overloading in the SLOT1BIN macro definition:
https://github.com/python/cpython/blob/v3.6.1/Objects/typeobject.c#L5796
There's also an explicit test for the arithmetic operation behaviour in
Lib/test/test_descr.py. In short, I doubt this was ever a bug: everything
points to this being a deliberate design decision. I hope someone on
python-ideas can elaborate on the rationale behind that design decision (and
also on why that rationale doesn't apply to comparisons).
In contrast, it does seem plausible to me that the *comparison* failure to
check for an explicit override may have been accidental.
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