title: Is there a Python dict without values?
body: <p>Instead of this:</p>
  
  <pre><code>a = {"foo": None, "bar": None}
  </code></pre>
  
  <p>Is there a way to write this?</p>
  
  <pre><code>b = {"foo", "bar"}
  </code></pre>
  
  <p>And still let <code>b</code> have constant time access (i.e. not a Python set, which cannot be keyed into)?</p>