exist.py

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

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

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