title: Example of set subtraction in python
body: <p>I'm taking a data structures course in Python, and a suggestion for a solution includes this code which I don't understand.</p>
  <p>This is a sample of a dictionary:</p>
  <pre><code>vc_metro = {
      'Richmond-Brighouse': set(['Lansdowne']),
      'Lansdowne': set(['Richmond-Brighouse', 'Aberdeen'])
  }
  </code></pre>
  <p>It is suggested that to remove some of the elements in the value, we use this code:</p>
  <pre><code>vc_metro['Lansdowne'] -= set(['Richmond-Brighouse'])
  </code></pre>
  <p>I have never seen such a structure, and using it in a basic situation such as:</p>
  <pre><code>my_list = [1, 2, 3, 4, 5, 6]
  other_list = [1, 2]
  my_list -= other_list
  </code></pre>
  <p>doesn't work. Where can I learn more about this recommended strategy?</p>