Tim Chase <python.list <at> tim.thechases.com> writes: > On 2013-03-06 22:20, Roy Smith wrote: > > I stumbled upon an interesting bit of trivia concerning lists and > > list comprehensions today. > > A little testing > shows that this can be rewritten as > > my_objects = list(iter(my_query_set)) > > which seems to then skip the costly __len__ call. Performance geeks > are welcome to time it against the list-comprehension version > > class Foo(object): > def __init__(self): > self.items = range(10) > def __iter__(self): > return iter(self.items) > def __len__(self): > print "Calling costly __len__" > return len(self.items) >
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. In most cases, such a workaround will not be feasible. Why should iter(QuerySet()) have a faster __len__() method defined than QuerySet() itself. Most likely, iter(QuerySet()) just returns self anyway? Best, Wolfgang -- http://mail.python.org/mailman/listinfo/python-list
