3 ms·
This is because the actual translation of `a + b` is a little more complicated. It's something like this: if '__add__' in a.__dict__: try: r
by bilboa 8y ago
This is because the actual translation of `a + b` is a little more complicated. It's something like this:
if '__add__' in a.__dict__:
try:
return a.__add__(b)
except NotImplemented:
if '__radd__' in b.__dict__:
return b.__radd__(a)
elif '__radd__' in b.__dict__:
return b.__radd__(a)
raise TypeError('unsupported operand type(s) for +: '{}' and '{}'.format(type(a), type(b))
In particular, the runtime seems to directly look for '__add__' in the object's __dict__, rather than just trying to invoke `__add__`, so your `__getattribute__` method isn't enough to make it work. If you add an actual `__add__` method to A your example will work.
- chriswarbo 8y agoYeah, I had a feeling the real 'expansion' would be more involved. I didn't know about the lookup in __dict__, that's good to know. I'm reminded of PHP, where (at least in version 5.*) we could write: $myObject->foo = function() { return "hello world"; }; $x = $myObject->foo; $x(); // Hello world $myObject->foo(); // Error: no such method 'foo' (Taken from an old comment https://news.ycombinator.com/item?id=8119419 https://news.ycombinator.com/item?id=8119419 )
- bilboa 8y agoI found the official docs on how python special methods are looked up. https://docs.python.org/3/reference/datamodel.html#special-lookup https://docs.python.org/3/reference/datamodel.html#special-l... Apparently the runtime is even more picky than I showed. The method has to be defined on the object's type, not in the object's instance dictionary. So, really the lookup is something like: if hasattr(type(a), '__add__'): The link I provided explains the rationale for bypassing the instance dictionary and `__getattribute__` method.