input: Write a function that identifies the longest word in a given sentence. If the sentence is empty then return an empty string.
  
  def find_longest_word(sentence):
      if not sentence:
          return ""
      words = sentence.split()
      # initialise longest_word variable with first word in the list
      longest_word = words[0]
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word
  
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("Hello world") == "Hello"
  assert find_longest_word("") == ""
  assert find_longest_word("Python is fun") == "Python"
  assert find_longest_word("Quick brown fox jumps over the lazy dog") == "Quick"
output: def find_longest_word(sentence):
      if not sentence:
          return ""
      words = sentence.split()
      longest_word = words[0]
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word