title: python dict.update vs. subscript to add a single key/value pair
body: <p>Every semester I have at least one Python student who uses <code>dict.update()</code> to add a single key/value pair, viz.:</p>
  
  <pre><code>mydict.update({'newkey':'newvalue'})
  </code></pre>
  
  <p>instead of</p>
  
  <pre><code>mydict['newkey'] = 'newvalue'
  </code></pre>
  
  <p>I don't teach this method and I don't know where they're finding examples of this, but I tell them not to do it because it's less efficient (presumably creates a new 2-element dict) and because it's nonstandard.  </p>
  
  <p>Honestly, I can understand the desire to use a visible method rather than this syntax - it perhaps feels more consistent with other method calls. But I think it looks like a newbie approach.  </p>
  
  <p>Is there any wisdom anyone has to offer on this point?  </p>