On 3/10/2013 9:05 AM, Roy Smith wrote:
In article <roy-572c99.22201106032...@70-1-84-166.pools.spcsdns.net>,
Roy Smith <r...@panix.com> wrote:
The problem is, QuerySets have a __len__() method. Calling it is a lot
faster than iterating over the whole query set and counting the items,
but it does result in an additional database query, which is a lot
slower than the list resizing! Writing the code as a list comprehension
prevents list() from trying to optimize when it shouldn't!
Hmmm, I think I've found a good solution.
It turns out, we don't actually use QuerySet in our models. We've
defined our own QuerySet subclass which adds a few convenience methods.
Adding
def __len__(self):
raise NotImplemented
to our subclass should do the job. It looks like list() respects that,
calls __iter__(), and does the right thing. I can't find any place
where that behavior for list() is documented,
It is a cpython implementation detail that probably has changed with the
versions.
but it's logical and experimentally, it seems to work.
Can anybody see any downside to this?
No. Give it a try.
--
Terry Jan Reedy
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