title: How to set a cell to NaN in a pandas dataframe
body: <p>I'd like to replace bad values in a column of a dataframe by NaN's.</p>
  
  <pre><code>mydata = {'x' : [10, 50, 18, 32, 47, 20], 'y' : ['12', '11', 'N/A', '13', '15', 'N/A']}
  df = pd.DataFrame(mydata)
  
  df[df.y == 'N/A']['y'] = np.nan
  </code></pre>
  
  <p>Though, the last line fails and throws a warning because it's working on a copy of df. So, what's the correct way to handle this? I've seen many solutions with iloc or ix but here, I need to use a boolean condition.</p>