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On Sat, Feb 14, 2015 at 11:12 AM, Ethan Furman <ethan@stoneleaf.us> wrote:
The current design of Python guarantees that an object always gets a setattr or setitem when one of its elements is assigned to. That's an important property, for the reasons I suggested above. So any change would have to preserve that property. And skipping assignment when __iadd__ returns self would not preserve that property. So it's not just backward-incompatible, it's bad.
--> some_var = ([1], 'abc') --> tmp = some_var[0] --> tmp += [2, 3] --> some_var ([1, 2, 3], 'abc')
In that example, 'some_var' is modified without its __setitem__ ever being called.
not really -- an object in the some_var is modified -- there could be any number of other references to that object -- so this is very much how python works. The fact that you can't directly use augmented assignment on an object contained in an immutable is not a bug, but it certainly is a wart -- particuarly since it will raise an Exception AFTER it has, in fact, performed the operation requested. I have argued that this never would have come up if augmented assignment were only used for in-place operations, but then we couldn't used it on integers, which was apparently desperately wanted ;-) (and the name "augmented assignment" would be a good name, either...). I don't know enough about how this all works under the hood to know if it could be made to work, but it seems the intention is clear here: object[index] += something. is a shorthand for: tmp = object[index] tmp += something or in a specific case: In [66]: t = ([], None) In [67]: t[0].extend([3,4]) In [68]: t Out[68]: ([3, 4], None) Do others agree that this, in fact, has an unambiguous intent? And that it would be nice if it worked? OR am I missing something? -Chris -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception Chris.Barker@noaa.gov