title: calculate mean or variance for subset of pandas DF column
body: <p>I have a gigantic pandas DF with a bunch of columns in it. I want to calculate mean and variance for a subset of three columns. Is there an easy way to do this without creating a whole new dataframe? I get all NaN whenever I try to use something like this: </p>
  
  <pre><code>DF['means']=np.mean(DF.A, DF.B, DF.C)
  </code></pre>
  
  <p>or </p>
  
  <pre><code>DF['means']=DF[['A','B','C','D']].mean(axis=0)
  </code></pre>
  
  <p>Thanks!</p>