title: pandas converts float64 to int
body: <p>I am trying to convert the dtype of a column (<code>A</code>) in a <code>dataframe</code> from <code>float64</code> to <code>int</code>, </p>
  
  <pre><code>df['A'].astype(numpy.int64)
  </code></pre>
  
  <p>but after that, <code>A</code> still gets <code>float64</code> as dtype. I am wondering how to resolve the issue.</p>