On Sat, Mar 8, 2014 at 5:40 PM, Gregory Ewing <greg.ew...@canterbury.ac.nz> wrote: > Ian Kelly wrote: >> >> 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? > > > In this case it will fail to catch what is probably an error, > but you can't expect the language to find all your bugs for > you. If you wrote the same bug this way: > > filter[25].extend([15, 17, 23]) > > it wouldn't be caught either. > > What's happening is that we're trying to use the syntax > a += b to mean two different things: > > 1) Shorthand for a = a + b > > 2) A way of expressing an in-place modification, such > as a.extend(b) > > Case (2) is not really an assignment at all, so arguably > it shouldn't require the LHS to support assignment.
In my view the second one is wrong. a += b should be understood as being equivalent to a = a + b, but with the *possible* and by no means guaranteed optimization that the operation may be performed in-place. In fact, if you read the documentation for lists, you may notice that while they clearly cover the + operator and the extend method, they do not explicitly document the list class's += operator. So although I'm not entirely sure whether it is intentional or not, and I would be quite surprised if some implementation were actually to differ on this point, the language does *not* from what I can see guarantee that the += operator on lists is equivalent to calling .extend. That having been said, code that uses += and relies on the operation to be performed in-place should be considered buggy. If you need the operation to be performed in-place, then use in-place methods like list.extend. If you need the operation not to be performed in-place, then use a = a + b. If you're ambivalent on the in-place issue and just want to write polymorphic code, that's when you should consider using +=. -- https://mail.python.org/mailman/listinfo/python-list
