title: How do I extract a sub-array from a numpy 2d array?
body: <p>I'd like to extract a numpy array with a specified size from a numpy 2d array--essentially I want to crop the array.
  For example, if have a numpy array like this:</p>
  
  <pre><code>([1,2,3],
   [4,5,6],
   [7,8,9])
  </code></pre>
  
  <p>I'd like to extract a 2x2 from it and the result should be:</p>
  
  <pre><code>([1,2],
   [4,5])
  </code></pre>
  
  <p>How can I do that?</p>