new_contents: # Define a procedure, find_last, that takes as input
  # two strings, a search string and a target string,
  # and returns the last position in the search string
  # where the target string appears, or -1 if there
  # are no occurrences.
  # 定义一个名叫find_last的函数，传入两个参数均为字符串，一个是搜索字符串，另一个是目标字符串。
  # 功能实现：目标字符串在搜索字符串中最后一次出现的位置；如果目标字符串从未出现过，则返回 -1。
  #
  
  def find_last(search_str, target_str):
      start_index = search_str.find(target_str)
      if start_index == -1: 
          return start_index   
      
      while start_index != -1: 
          last_index = start_index 
          start_index = search_str.find(target_str, start_index + 1)
      return last_index
  
subject: Create 2_string_search_operation.py