Alf P. Steinbach wrote: > <test lang="py3"> > >>> t = ([], [], []) > >>> t > ([], [], []) > >>> t[0] += ["blah"] > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: 'tuple' object does not support item assignment > >>> t > (['blah'], [], []) > >>> _ > </test> > > Yep, it matters. > > Is this change-but-raise-exception a bug?
No. a[0] += b translates to a.__setitem__(0, a.__getitem__(0).__iadd__(b)) assuming a[0] has an __iadd__() method. It should be obvious that only the the last operation, the outer a.__setitem__(...), will fail here. A possible fix might be a changed order of evaluation: _internal_set = a.__setitem__ _internal_set(0, a.__getitem__(0).__iadd__(b)) I don't know if there are arguments against this other than increased compiler complexity. Or one could argue that a += b should have been implemented as a = a + b or a = a.__add__(b) which is currently used as the fallback when there is no __iadd__() method and which gives a more intuitive behaviour at the cost of a greater overhead. But it's a little late for that discussion, for that language. Peter -- http://mail.python.org/mailman/listinfo/python-list
