count.py

#  Don't waste a query if you are using the queryset
books = Book.objects.filter(..)
if len(books) > 5:
  do_stuff_with_books(books)

#  If you aren't using the queryset use count
books = Book.objects.filter(..)
if books.count() > 5:
  do_some_stuff()

#  But never
if len(Book.objects.filter(..)) > 5:
  do_some_stuff()