title: "None" in Python
body: <p>So, I have this pretty simple function (I'm a novice still).</p>
  
  <p>And it goes like:</p>
  
  <pre><code>def add_num(x,y,z=None):
      if z == None:
          return x+y
      else:
          return x+y+z
  
  print(add_num(1,2))
  print(add_num(1,2,3))
  </code></pre>
  
  <p>So my question is, when I notice that if there is no third variable, then it is accepted. So does "None" mean basically mean that it is ok to have no value attached to a variable if "variable=None."  Just confirming!  Thanks! </p>