input: Write a function to identify the longest word in a given sentence. Return the longest word as a string. If the sentence is empty, return an empty string.
  def find_longest_word(sentence):
      words = sentence.split()
      # initialise longest_word variable with 0
      longest_word = ""
      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("Python programming is fun.") == "programming"
  assert find_longest_word("A quick brown fox jumps over the lazy dog.") == "quick"
  assert find_longest_word("") == ""
  assert find_longest_word("SingleWord") == "SingleWord"
output: def find_longest_word(sentence):
      words = sentence.split()
      longest_word = ""
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word