title: Why python allows tuple as key for dictionary
body: <p>tuples in python can have elements of different type. For example:</p>
  
  <pre><code>tup1 = ('physics', 'chemistry', 1997, 2000);
  tup2 = (1, 2, 3, 4 );
  </code></pre>
  
  <p>When used for keys in dictionary, how does python decide the size of key when the element size are varied?</p>