input: Write a function to identify the longest word in a given sentence. Return the longest word found. If the sentence is empty, return an empty string.
  
  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
  
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("Quick brown fox jumps over the lazy dog.") == "Quick"
  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