title: How to lowercase a pandas dataframe string column if it has missing values?
body: <p>The following code does not work. </p>
  
  <pre><code>import pandas as pd
  import numpy as np
  df=pd.DataFrame(['ONE','Two', np.nan],columns=['x']) 
  xLower = df["x"].map(lambda x: x.lower())
  </code></pre>
  
  <p>How should I tweak it to get xLower = ['one','two',np.nan] ?
  Efficiency is important since the real data frame is huge.</p>