input: Write a function to count the number of words in a given string. The function should return the number of words as an integer.
  
  def count_words(s: str) -> int:
      return len(s.split())
  
  assert count_words('Hello world, this is a sentence.') == 5
  assert count_words('This is another example sentence.') == 5
  assert count_words('Another single word.') == 3
  assert count_words('Single.') == 1
  assert count_words('') == 0
output: def count_words(s: str) -> int:
      return len(s.split())