title: How can I check that a list has one and only one truthy value?
body: <p>In python, I have a list that should have <strong>one and only one</strong> truthy value (that is, <code>bool(value) is True</code>). Is there a clever way to check for this? Right now, I am just iterating across the list and manually checking:</p>
  
  <pre><code>def only1(l)
      true_found = False
      for v in l:
          if v and not true_found:
              true_found=True
          elif v and true_found:
               return False #"Too Many Trues"
      return true_found
  </code></pre>
  
  <p>This seems inelegant and  not very pythonic. Is there a cleverer way to do this? </p>