title: Python string 'in' operator implementation algorithm and time complexity
body: <p>I am thinking of how the <code>in</code> operator implement, for instance</p>
  
  <pre><code>&gt;&gt;&gt; s1 = 'abcdef'
  &gt;&gt;&gt; s2 = 'bcd'
  &gt;&gt;&gt; s2 in s1
  True
  </code></pre>
  
  <p>In CPython, which algorithm is used to implement the string match, and what is the time complexity? Is there any official document or wiki about this?</p>