title: find() method in python3
body: <p>I'm unable to understand what's the second argument in the find method in python3 is for.I searched it online but couldn't get a satisfactory explanation.For ex, please explain the use of the number 5 ,20 in the following code.</p>
  
  <pre><code>&gt;&gt;&gt; str = "this is a string"
  &gt;&gt;&gt; str.find("is", 5)
  5
  &gt;&gt;&gt; str.find("is", 20)
  -1
  </code></pre>