[Tutor] Conditional attribute access / key access

[Knacktus]

Am 30.08.2010 17:53, schrieb Francesco Loffredo:
>
> Two questions and one doubt for you:
> 1- How many "generations" do you want to keep in a single item (call it
> dictionary or list, or record, whatever)? I mean, what if some children
> have children too, and some of those have more children, etc ?
There's always one level (generation) of children in an item. An item 
can have zero or more direct children. And a lot of grandchildren and 
grandgrandchildren etc. The item-structure represent an assembly 
hierarchy of the parts of a car. So overall the structure can be up to 
about 20 levels "deep" and consist of up to 200000 items overall, where 
the application needs to handle several structures.
>
> 2- Are you SURE that there are no circular references in your database?
> In your example, what if item_3 was
> item_3 = {"id": 3, "name": "child_2", "children_ids": [6, 1, 8]}? Is't
> it possible that those recursion limit problems you had could come from
> some circular reference in your data?
That's a good hint. But the recursion limit doesn't come from that (the 
test data actually had no children. I used a single instance of my dict.)
>
> d- If the number of data items is really huge, are you sure that you
> want to keep the whole family in memory at the same time? It depends on
> the answer you gave to my question #1, of course, but if retrieving an
> item from your database is quick as it should be, you could use a query
> to resolve the references on demand, and you wouldn't need a special
> structure to hold "the rest of the family". If the retrieval is slow or
> difficult, then the creation of your structure could take a significant
> amount of time.
One thing is, that I have to do some additional calculations when 
resolving the structure. The items will get some kind of 
labels/conditions and versions, further, when resolving the structure a 
set of rules for those conditions is given. At my first shot I'll have 
to do those calculations in the Python code (even if it would be very 
wicked to do stuff like that with SQL). So, I will always have a large 
number of items in memory, as I don't want to call the database for each 
structure-level I want to expand. Also, I'm using a pyqt-treeview 
(AbstractItemModel) for a client-site gui. For this purpose I need to 
create an additional structure, as in the original data items can have 
more than one parent, which is not permitted in the model for the treeview.
The whole idea of replacing the id-references to object-references is to 
enhance performance and make the application code easier.

Thanks for the feedback so far.
>
> Hope this helps,
> Francesco
>
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