title: Python - curly braces in type hints
body: <p>What do these mean?</p>
  
  <pre><code> def f(a: {int, float}):
      pass
  </code></pre>
  
  <p>I've seen this syntax used in some standard Python modules when fetching documentation via PyCharm, and I have no idea what it means. What's the hinted type for <code>a</code> in my example? What types can I pass to this function?</p>
  
  <p>The particular example where I've seen this is in <code>tkinter</code>'s <code>Frame</code> <code>__init__</code> method, where the <code>master</code> parameter is of type <code>{tk, _w}</code>.</p>