title: Checking user input using isnan function of NumPy
body: <p>I'm trying to use NumPy to check if user input is numerical. I've tried using:</p>
  
  <pre><code>import numpy as np
  
  a = input("\n\nInsert A: ")
  
  if np.isnan(a):
      print 'Not a number...'
  else:
      print "Yep,that's a number"
  </code></pre>
  
  <p>On its own t works fine, however when I embed it into a function such as in this case:</p>
  
  <pre><code>import numpy as np
  
  def test_this(a):   
      if np.isnan(a):
          print '\n\nThis is not an accepted type of input for A\n\n'
          raise ValueError
      else:
          print "Yep,that's a number"
  
  a = input("\n\nInsert A: ")
  
  test_this(a)
  </code></pre>
  
  <p>Then I get a <code>NotImplementationError</code> saying it isn't implemented for this type, can anyone explain how this is not working? </p>