Python: What is the best way to check if a list is empty?

a = []

if not a:
  print "List is empty"

if not seq:
if seq:

if len(seq)
if not len(seq)

a = []
if a:
     print "not empty"

if not a:
    print "The list is empty or null"

if a == []:
   print "The list is empty."

if len(li) == 0:
    print 'the list is empty'