zip as iterator and bad/good practices

[Steven D'Aprano]

On Sat, 13 Jun 2015 13:48:45 +0100, Oscar Benjamin wrote:

> On 13 June 2015 at 08:17, Steven D'Aprano
> <steve+comp.lang.python at pearwood.info> wrote:

>> But an easier way is:
>>
>> Test = [1, 2]
>> Test.extend(Test)
>> print(Test)
> 
> I can't see anything in the docs that specify the behaviour that occurs
> here. 

Neither do I, but there is a test for it:

        a.extend(a)
        self.assertEqual(a, self.type2test([0, 0, 1, 0, 0, 1]))

https://hg.python.org/cpython/file/a985b6455fde/Lib/test/list_tests.py

> If I change it to
> 
>     Test.extend(iter(Test))
> 
> then it borks my system in 1s after consuming 8GB of RAM (I recovered
> with killall python in the tty).

The reason that fails should be obvious: as new items keep getting added 
to Test, the iterator likewise sees more items to iterate over. I don't 
know if this is documented, but you can see what happens here:

py> L = [10, 20]
py> it = iter(L)
py> L.append(next(it)); print L
[10, 20, 10]
py> L.append(next(it)); print L
[10, 20, 10, 20]
py> L.append(next(it)); print L
[10, 20, 10, 20, 10]
py> L.append(next(it)); print L
[10, 20, 10, 20, 10, 20]


So as Test.extend tries to iterate over iter(Test), it just keeps growing 
as more items are added to Test.


-- 
Steven D'Aprano