On Thu, Mar 7, 2013 at 4:22 AM, Wolfgang Maier
<wolfgang.ma...@biologie.uni-freiburg.de> 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()
> In
> most cases, such a workaround will not be feasible. Why should
> iter(QuerySet())
> have a faster __len__() method defined than QuerySet() itself.
iter(QuerySet()) should not have any __len__ method defined at all,
which is why the optimization would be skipped.
> Most likely,
> iter(QuerySet()) just returns self anyway?
But on this point, you are correct. The mongoengine QuerySet.__iter__
method is defined as:
def __iter__(self):
self.rewind()
return self
This is unfortunate design. Not only does it mean that the iterator's
__len__ method cannot be trusted (what should the __len__ of a
partially exhausted iterator return?), but it also means that requesting
an iterator over the QuerySet will also silently invalidate any
existing iterators.
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