title: cache a dataframe in pyspark
body: <p>I want to know more precisely about the use of the method cache for dataframe in pyspark</p>
  
  <p>When I run <code>df.cache()</code> it returns a dataframe. 
  Therefore, if I do <code>df2 = df.cache()</code>, which dataframe is in cache ? Is it <code>df</code>, <code>df2</code>, or both ? </p>