input: Write a function to count the number of words in a given string. The function should return the number of words as the output.
  assert word_count('Hello world, this is a sentence.')==5
  assert word_count('This is another sentence with seven words.')==7
  assert word_count('Single.')==1
  assert word_count('')==0
  assert word_count('Two words.')==2
output: def word_count(s):
      return len(s.split())