title: pandas to_csv , number of unique record reduces
body: <p>I'm writing a pandas dataframe to a csv file. However I can find that number of unique ids are decreasing, but total number of rows remains the same</p>
  
  <p>Below you can find the code:</p>
  
  <p>checking total count and unique count of grouped_test</p>
  
  <pre><code> grouped_test['fullVisitorId'].nunique() , grouped_test['fullVisitorId'].count() 
  (639230, 639230)
  </code></pre>
  
  <p>Submitting to a csv file</p>
  
  <pre><code>grouped_test.to_csv('result.csv',index=False)
  </code></pre>
  
  <p>Reading the same csv file again</p>
  
  <pre><code>result = pd.read_csv('result.csv')
  </code></pre>
  
  <p>I got below warning when reading the file</p>
  
  <pre><code>py:2785: DtypeWarning: Columns (0) have mixed types. Specify dtype option on import or set low_memory=False.
    interactivity=interactivity, compiler=compiler, result=result)
  </code></pre>
  
  <p>checking the count</p>
  
  <pre><code>result['fullVisitorId'].nunique() , result['fullVisitorId'].count()
  (638932, 639230)
  </code></pre>
  
  <p>You can find the number of unique id decreases, any idea what could be the issue.</p>
  
  <p>Below you can find some sample values of Ids. Does the length/datatype of the id has anything to do with it?</p>
  
  <pre><code>9844437513179813866
  643697640977915618
  </code></pre>