title: How to extract all columns but one from an array (or matrix) in python?
body: <p>Given a numpy 2d array (or a matrix), I would like to extract all the columns but the i-th.</p>
  
  <p>E. g. from</p>
  
  <pre><code>1 2 3 4
  2 4 6 8
  3 6 9 12
  </code></pre>
  
  <p>I would like to have, e.g.</p>
  
  <pre><code>1 2 3
  2 4 6
  3 6 9
  </code></pre>
  
  <p>or</p>
  
  <pre><code>1 2 4
  2 4 8
  3 6 12
  </code></pre>
  
  <p>I cannot find a pythonic way to do this. I now that you can extract given columns by simply</p>
  
  <pre><code>a[:,n]
  </code></pre>
  
  <p>or </p>
  
  <pre><code>a[:,[n,n+1,n+5]]
  </code></pre>
  
  <p>But what about extracting all of them but one?</p>