title: Python pandas: change one column conditional on another
body: <p>How can I reverse the sign in the quantity column whenever the side column has a sell? All other values should not be changed. The following is simply not working, it has no effect.</p>
  
  <pre><code>df[df['side'] == 'Sell']['quantity'] = df[df['side'] == 'Sell']['quantity'] * -1
  </code></pre>