[Tutor] How to override getting items from a list for iteration

[Steven D'Aprano]

On 11/02/13 01:32, Walter Prins wrote:
> Hello,
>
> I have a program where I'm overriding the retrieval of items from a list.
>   As background:
[...snip interesting but irrelevant background...]

> Here's a test application that demonstrates the issue:
[...snip overly complicated application...]


Here is a much simpler demonstration:


py> class MyList(list):
...     def __getitem__(self, i):
...             print "Checking item %d" % i
...             return super(MyList, self).__getitem__(i)
...
py> x = MyList([2, 4, 8, 16])
py> x[3]
Checking item 3
16
py> for i in x:
...     print "got %s" % i
...
got 2
got 4
got 8
got 16



> What's the best way to fix this problem?  Do I need to maybe override
> another method, perhaps provide my own iterator implementation?


Pretty much. For lists, the __iter__ method returns a listiterator object
which efficiently iterates over the list items without calling __getitem__.

Try overriding your class' __iter__ method:

     def __iter__(self):
         for i in range(len(self)):  # use xrange in Python 2
             yield self[i]


(untested).


> For that
> matter, why doesn't iterating over the list contents fall back to calling
> __getitem__?


Probably as an optimization.



-- 
Steven