title: Sum columns in a pandas dataframe which contain a string
body: <p>I am trying to do something relatively simple in summing all columns in a pandas dataframe that contain a certain string.  Then making that a new column in the dataframe from the sum.  These columns are all numeric float values...</p>
  
  <p>I can get the list of columns which contain the string I want</p>
  
  <p><code>StmCol = [col for col in cdf.columns if 'Stm_Rate' in col]</code></p>
  
  <p>But when I try to sum them using: 
  <code>cdf['PadStm'] = cdf[StmCol].sum()</code></p>
  
  <p>I get a new column full of "nan" values.</p>