Roy Smith wrote:
If I've got an object foo, and I execute:
foo.bar += baz
exactly what happens if foo does not have a 'bar' attribute? It's
pretty clear that foo.__getattr__('bar') gets called first, but it's a
little murky after that. Assume for the moment that foo.__getattr__
('bar') returns an object x. I think the complete sequence of calls
is:
foo.__getattr__('bar') ==> x
x.__add__(baz) ==> y
foo.__setattr__('bar', y)
but I'm not 100% sure. It would be nice if it was, because that would
let me do some very neat magic in a system I'm working on :-)
How would things change if X defined __iadd__()?
The semantic of the in-place operator is something like:
x += y
becomes
x = x.__iadd__(y)
thus
foo.bar += baz
becomes
foo.bar = foo.bar.__iadd__(baz)
So the call sequence is,
foo.__getattr__('bar') ==> x
x.__iadd__(baz) ==> y
foo.__setattr__('bar', y)
the default definition of object.__iadd__ is something like this:
def __iadd__(self, other):
# this calls self.__add__ or other.__radd__ according to the
# operator call rule, may call __coerce__ or any other magics
# in operator calling
return self + other
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