title: Pandas read_csv: low_memory and dtype options
body: <pre><code>df = pd.read_csv('somefile.csv')
  </code></pre>
  <p>...gives an error:</p>
  <blockquote>
  <p>.../site-packages/pandas/io/parsers.py:1130:
  DtypeWarning: Columns (4,5,7,16) have mixed types.  Specify dtype
  option on import or set low_memory=False.</p>
  </blockquote>
  <p>Why is the <code>dtype</code> option related to <code>low_memory</code>, and why might <code>low_memory=False</code> help?</p>