input: Write a function to count the number of words in a given string. If the string is empty, return 0.
  
  def count_words(string):
      if string == "":
          return 0
      return len(string.split())
  
  assert count_words('Hello world, this is a sentence.') == 5
  assert count_words('') == 0
  assert count_words('One') == 1
  assert count_words('This has four words') == 4
  assert count_words('Python is fun') == 3
output: def count_words(string):
      if string == "":
          return 0
      return len(string.split())