title: How to copy a large dataframe to a smaller one?
body: <p>Probably have been asked before but I could not find anything. Learning Pandas and I thought the learning curve would not be so steep - I was wrong =)
  I am importing a csv file with over 20 million rows. Problem is that it is making it hard to work with because of the size.
  Just altering a column to use proper dateTime takes ages.</p>
  
  <p>So my first idea was to tail the last 100 rows -</p>
  
  <pre><code>shortDataFrame = longDataFrame.tail(100).copy()
  </code></pre>
  
  <p>However this gives me some strange KeyError: 0L when I try to check the type for certain columns.
  This gives errors-</p>
  
  <pre><code>type(shortDataFrame['Time'][0])
  </code></pre>
  
  <p>This works and gives no errors on original long dataFrame</p>
  
  <pre><code>type(longDataFrame['Time][0])
  </code></pre>
  
  <p>So how to make a large dataframe into a smaller one to make it more manageable?</p>