title: update method of python dictionary did not work
body: <p>I have two dictionaries.</p>
  
  <pre><code>a = {"ab":3, "bd":4}
  b = {"cd":3, "ed":5}` 
  </code></pre>
  
  <p>I want to combine them to <code>{'bd': 4, 'ab': 3, 'ed': 5, 'cd': 3}</code>. </p>
  
  <p>As <a href="https://stackoverflow.com/questions/577234/python-extend-for-a-dictionary">this</a> says, <code>a.update(b)</code> can complete it. But when I try, I get:</p>
  
  <pre><code>type(a.update(b)) #--&gt; type 'NoneType'
  </code></pre>
  
  <p>Would anyone like to explain it to me why I cannot gain a dict type?</p>
  
  <p>I also tried this, and it did well:</p>
  
  <pre><code>type(dict(a,**b)) #--&gt;type 'dict'
  </code></pre>
  
  <p>What is the difference between these two methods and why did the first one not work?</p>