On Tue, 01 Dec 2015 14:44:38 -0600, Ian Kelly wrote: > On Tue, Dec 1, 2015 at 2:32 PM, Denis McMahon <denismfmcma...@gmail.com> > wrote: >> On Tue, 01 Dec 2015 03:32:31 +0000, MRAB wrote: >> >>> In the case of: >>> >>> tup[1] += [6, 7] >>> >>> what it's trying to do is: >>> >>> tup[1] = tup[1].__iadd__([6, 7]) >>> >>> tup[1] refers to a list, and the __iadd__ method _does_ mutate it, but >>> then Python tries to put the result that the method returns into >>> tup[1]. >>> That fails because tup itself is a tuple, which is immutable. >> >> I think I might have found a bug: >> >> $ python Python 2.7.3 (default, Jun 22 2015, 19:33:41) >> [GCC 4.6.3] on linux2 Type "help", "copyright", "credits" or "license" >> for more information. >>>>> tup = [1,2,3],[4,5,6] >>>>> tup >> ([1, 2, 3], [4, 5, 6]) >>>>> tup[1] >> [4, 5, 6] >>>>> tup[1] += [7,8,9] >> Traceback (most recent call last): >> File "<stdin>", line 1, in <module> >> TypeError: 'tuple' object does not support item assignment >>>>> tup[1] >> [4, 5, 6, 7, 8, 9] >>>>> tup >> ([1, 2, 3], [4, 5, 6, 7, 8, 9]) >>>>> quit() > > No, that's the expected result. As MRAB wrote, the list *is* mutated > when its __iadd__ method is called. The TypeError happens afterward when > the assignment is attempted.
The assignment succeeds. That's imo a bug. If it's a TypeError to try and assign a value to tup[1], then tup[1] should not allow the mutated list to be assigned. -- Denis McMahon, denismfmcma...@gmail.com -- https://mail.python.org/mailman/listinfo/python-list
