title: How to return a dataframe with an added column without overwriting the previous dataframe?
body: <p>I want to create a dataframe that is equal to an existing dataframe with an added column.
  For example, say I have an existing dataframe df1, and I want to create a new dataframe df2 that is just df1 with a new column of 3s. How could I do in Pandas so without affecting df1 at all?</p>