On Fri, Mar 7, 2014 at 7:17 PM, Gregory Ewing
<greg.ew...@canterbury.ac.nz> wrote:
> Here's another idea: If the __iadd__ method returns the
> same object, *and* the LHS doesn't have a __setitem__
> method, then do nothing instead of raising an exception.
Maybe it doesn't have a __setitem__ because the object that was
retrieved is computed rather than stored, and the result of the
__iadd__ will simply be discarded. Somewhat contrived example:
class LessThanFilter:
def __init__(self, the_list):
self._the_list = the_list
def __getitem__(self, bound):
return [x for x in self._the_list if x < bound]
filter = LessThanFilter([10, 20, 30, 40, 50])
filter[25] += [15, 17, 23]
Should that last line not raise an exception? The __iadd__ call will
return the same object, and the LHS doesn't have a __setitem__ method.
> I don't think that's a problem, because the use case
> being addressed is where the object performs in-place
> modification and always returns itself. Any object that
> doesn't return itself is not modifying in-place, even
> if the returned object happens to be equal to the
> original one.
I already mentioned this earlier in the thread, but a balanced binary
tree might implement += as node insertion and then return a different
object if the balancing causes the root node to change.
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