title: pandas dtype conversion from object to string
body: <p>I have a csv file that has a few columns which are numbers and few that are string. When I try <code>myDF.dtypes</code> it shows me all the string columns as <code>object</code>. </p>
  
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  <li><p>Someone asked a related question before <a href="https://stackoverflow.com/questions/21018654/strings-in-a-dataframe-but-dtype-is-object">here</a> about why this is done. Is it possible to recast the <code>dtype</code> from object to string?</p></li>
  <li><p>Also, in general, is there any easy way to recast the <code>dtype</code> from <code>int64</code> and <code>float64</code> to <code>int32</code> and <code>float32</code> and save on the size of the data (in memory / on disk)?</p></li>
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