title: pandas dataframe, copy by value
body: <p>I noticed a bug in my program and the reason it is happening is because it seems that pandas is copying by reference a pandas dataframe instead of by value. I know immutable objects will always be passed by reference but pandas dataframe is not immutable so I do not see why it is passing by reference. Can anyone provide some information? </p>
  
  <p>Thanks!
  Andrew</p>