title: why this word_dict = {word: None for word in word_list} so fast
body: <p>this code work so fast</p>
  <pre class="lang-py prettyprint-override"><code>with open('words.txt') as fd:
      word_list = fd.read().splitlines()
  
  
  word_dict = {word: None for word in word_list}   
  print(word_dict)
  
  def find_rev_pairs(word_dict):
      for word in word_dict:
          if word[::-1] in word_dict:
              print (word, word[::-1])
  
  find_rev_pairs(word_dict)
  </code></pre>
  <p>the problem in this line</p>
  <pre class="lang-py prettyprint-override"><code>word_dict = {word: None for word in word_list}   
  </code></pre>
  <p><strong>when I use it code work so fast if I use <code>word_list</code> directly code will be so slow</strong></p>