Re: list comprehension query

On Fri, May 3, 2013 at 2:32 PM, MikeKJ <mike.jones@paston.co.uk> wrote:
> I am using enumerate to find the position in a list but it doesnt seem to be
> working or throwing an error any ideas please?

"Not working" is not a very good description of what is going wrong.
If it did throw an error, not including it is also a bit of a pain..

>
> def detail(request, item_id):
> item = Item.objects.get(pk=item_id)
> list = []

You shouldn't use keywords like 'list' or 'dict' as variable names, it
(eventually) causes hard to solve issues.

> count = Item.objects.all().count()
> all = Item.objects.all()
> for y in all:
> list.append(y.id)

This whole section is better written:

item_list = list(Item.objects.all().values_list('id', flat=True))
count = len(item_list)

> get = (i for i,x in enumerate(list) if x==item_id)

First of all, item_id is probably at this point a string - it's come
from the URL, right?

item_id = int(item_id)

If you do list comprehensions with '()' you end up with a generator
function. If you do it with '[]', you get a proper list comprehension:

idx_list = [ idx for idx, list_item_id in enumerate(item_list) if
item_id == list_item_id ]

However, you are searching a list for a specific value, and all the
values in the list are distinct:

idx = item_list.index(item_id)

> for i in get:
> raise NameError(i)
> if i > 0:
> previous = list[i - 1]
> if i < (count-1):
> next = list[i + 1]
> return render_to_response('news/item_detail.html',{ 'item': item,},
> context_instance=RequestContext(request))
>
> cheers

The rest should work, apart from the (deliberate?) NameError, and the
fact that you have the index of your current item in your list in a
single variable.

This is not a particularly efficient way of finding the previous and
next items. I would simply hit the DB twice, rather than load every
item id into an array and search it:

previous = None
next = None
try:
previous = Item.objects.filter(pk__lt=item_id)[0:1].get()
except Item.DoesNotExist:
pass
try:
next = Item.objects.filter(pk__gt=item_id)[0:1].get()
except Item.DoesNotExist:
pass

Cheers

Tom

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