title: unknown python syntax (function_name[data type]) - want more info
body: <p>I'm looking for more info on this kind of syntax</p>
  
  <p>function_name[data type]</p>
  
  <p>I don't know what this is called so I hope someone here can point me in the right direction. I'm used to this kind of syntax with referencing values in list and dictionaries but not so much for function definitions.</p>
  
  <p>I came across this when following PyQt5 tutorials, specifically this one</p>
  
  <p><a href="http://zetcode.com/gui/pyqt5/widgets/" rel="nofollow noreferrer">http://zetcode.com/gui/pyqt5/widgets/</a></p>
  
  <p>which contains this line</p>
  
  <pre><code>sld = QSlider(Qt.Horizontal, self)
  ...
  sld.valueChanged[int].connect(...)
  </code></pre>
  
  <p>From the Qt5 documentation I can see that this is the definition of the signal function</p>
  
  <pre><code>valueChanged(int value)
  </code></pre>
  
  <p><a href="https://doc.qt.io/qt-5.11/qabstractslider.html#signals" rel="nofollow noreferrer">https://doc.qt.io/qt-5.11/qabstractslider.html#signals</a></p>
  
  <p>Thanks,</p>