On 12/1/2015 3:32 PM, Denis McMahon 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:
What you found is an specific example of what MRAB said in general above.
$ 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
The bug is trying to replace a member of a tuple. The correct code, to
avoid the exception while extending the list, is
tup[1].extend([7,8,9])
tup[1]
[4, 5, 6, 7, 8, 9]
--
Terry Jan Reedy
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