input: Write a function that reverses the order of words in a given string. The input is a string of words separated by spaces, and the output should be a string with the words in reverse order.
  
  def reverse_words(str):
      return " ".join(str.split()[::-1])
  
  assert reverse_words("Hello world")=="world Hello"
  assert reverse_words("Python is fun")=="fun is Python"
  assert reverse_words("The quick brown fox")=="fox brown quick The"
  assert reverse_words("SingleWord")=="SingleWord"
  assert reverse_words("")==""
output: def reverse_words(str):
      return " ".join(str.split()[::-1])