title: How to raise own error when one argument is missing?
body: <p>Hi I'm pretty new to Python and I've just started to learn about errors and exceptions.I have this function in a class that inserts a line at a given index called num.I know python will raise an error if no num is given but I want to raise my own error.How do I do that?This is what I tried. But the error raised is still the default error?</p>
  
  <pre><code> def insertNum(self, num, line):
      if num== None:
          raise Exception("Num not specified.")
      else:
          self.list.insert(num, line)
      return self.list
  </code></pre>