title: dropping rows from dataframe based on a "not in" condition
body: <p>I want to drop rows from a pandas dataframe when the value of the date column is in a list of dates. The following code doesn't work:</p>
  
  <pre><code>a=['2015-01-01' , '2015-02-01']
  
  df=df[df.datecolumn not in a]
  </code></pre>
  
  <p>I get the following error:</p>
  
  <blockquote>
    <p>ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().</p>
  </blockquote>