title: How to exclude values from pandas dataframe?
body: <p>I have two dataframes:</p>
  
  <p>1) customer_id,gender
  2) customer_id,...[other fields]</p>
  
  <p>The first dataset is an answer dataset (gender is an answer). So, I want to exclude from the second dataset those customer_id which are in the first dataset (which gender we know) and call it 'train'. The rest records should become a 'test' dataset.</p>