title: python thread to sequentially process data
body: <p>I want two thread, that's say <code>thread1</code> and <code>thread2</code>. <code>thread1</code> takes 'c++' as input, and output ['c', '+', '+'], put it in a list, that's say <code>res</code>;
  and <code>thread2</code> takes 'python' as input, and output ['p', 'y', 't', 'h', 'o', 'n'], put it in <code>res</code>. So after processing, res will be  [['c', '+', '+'], ['p', 'y', 't', 'h', 'o', 'n']], not [['p', 'y', 't', 'h', 'o', 'n'], ['c', '+', '+']]. The logic is like the following code, but I want a thread version code.</p>
  
  <pre><code>train_set = [
      'c++',
      'python',
  ]
  
  def to_list(item):
      return list(item)
  
  res = list(map(to_list, train_set))
  print(res) # [['c', '+', '+'], ['p', 'y', 't', 'h', 'o', 'n']]
  </code></pre>