input: Write a function to identify the longest word in a given sentence. Return the longest word as a string.
  def find_longest_word(sentence):
      words = sentence.split()
      # initialise longest_word variable with 0
      longest_word = 0
      for word in words:
          if len(word) > longest_word:
              longest_word = len(word)
              longest_word_str = word
      return longest_word_str
  
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("Python is a versatile programming language.") == "programming"
  assert find_longest_word("This is a simple test sentence.") == "sentence"
  assert find_longest_word("") == ""
output: def find_longest_word(sentence):
      words = sentence.split()
      longest_word_str = ""
      for word in words:
          if len(word) > len(longest_word_str):
              longest_word_str = word
      return longest_word_str