title: Selecting multiple columns in a Pandas dataframe
body: <p>How do I select columns <code>a</code> and <code>b</code> from <code>df</code>, and save them into a new dataframe <code>df1</code>?</p>
  <pre class="lang-none prettyprint-override"><code>index  a   b   c
  1      2   3   4
  2      3   4   5
  </code></pre>
  <p>Unsuccessful attempt:</p>
  <pre><code>df1 = df['a':'b']
  df1 = df.ix[:, 'a':'b']
  </code></pre>