Ian Kelly, 07.03.2013 18:31: > On Thu, Mar 7, 2013 at 9:20 AM, Christian Heimes wrote: >> Am 07.03.2013 17:00, schrieb Ian Kelly: >>> On Thu, Mar 7, 2013 at 4:22 AM, Wolfgang Maier wrote: >>>> Well, it skips the costly len() call because your iter(Foo()) returns >>>> iter(range()) under the hood and list() uses that object's __len__() >>>> method. >>> >>> Iterators do not generally have __len__ methods. >>> >>> >>> len(iter(range(10))) >>> Traceback (most recent call last): >>> File "<stdin>", line 1, in <module> >>> TypeError: object of type 'range_iterator' has no len() >> >> But iterators have a length hint method that are used for some >> optimizations and preallocations, too. >> >> >>> i = iter(range(10)) >> >>> i.__length_hint__() >> 10 >> >> See http://www.python.org/dev/peps/pep-0424/ > > Didn't know about that, thanks. Presumably a proper iter(QuerySet()) > object could implement __length_hint__ in an efficient manner rather > than by just calling the __len__ of the underlying QuerySet,
And how exactly would it do that, without either doing what __len__ does or reading the whole result set into memory? Stefan -- http://mail.python.org/mailman/listinfo/python-list
