title: np array as optional arguments
body: <p>I have a function that can accept two optional <code>np.array</code> as arguments. In case <em>both</em> are passed the function should do perform some task.</p>
  
  <pre><code>def f(some_stuff, this=None, that=None):
      ...do something...
      if this and that:
         perform_the_task()
  </code></pre>
  
  <p>This works as expected if none of the optional arguments are passed. If I pass a <code>np.array</code> then I obtain the error</p>
  
  <pre><code>ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
  </code></pre>
  
  <p>Is there more compact way of checking if the extra args are passed? I guess I can safely assume that <em>if</em> they are passed, <em>then</em> they are going to be <code>np.array</code>.</p>