title: Python syntax for "if a or b or c but not all of them"
body: <p>I have a python script that can receive either zero or three command line arguments. (Either it runs on default behavior or needs all three values specified.)</p>
  
  <p>What's the ideal syntax for something like:</p>
  
  <pre><code>if a and (not b or not c) or b and (not a or not c) or c and (not b or not a):
  </code></pre>
  
  <p>?</p>