title: Use of isinstance(variable,data_type_name) function for NoneType data type gone wrong
body: <p>Suppose a variable is defined:</p>
  
  <pre class="lang-py prettyprint-override"><code>a=None
  </code></pre>
  
  <p><br>
  On running below code:</p>
  
  <pre class="lang-py prettyprint-override"><code>print(isinstance(a,NoneType))
  </code></pre>
  
  <p>gave error: <strong>NameError</strong>: name 'NoneType' is not defined<br><br>
  For any other data type, its working correctly</p>
  
  <pre class="lang-py prettyprint-override"><code>print(isinstance(5,int), isinstance(4.0,float), isinstance(False,bool), isinstance("on_and_on",str), isinstance([1,"2",3.0,True,None],list), isinstance((1,"2",3.0,True,None),tuple), isinstance({4,1,3,2},set), isinstance({1:"a","apple":2,5.0:True},dict), isinstance(5-2j,complex))
  </code></pre>
  
  <p>Why?</p>