What is the difference between single bracket df["column"] and double bracket df[["column"]]
to be more specific, df['column'] returns only one column, but when you use df[['column']] you can call more than one column.
for example df[['column1','column2']] returns column1 and column2 from df
One is a pandas.core.series.Series and another is a dataframe pandas.core.frame.DataFrame. 
I have seen codes using them both. Is there a guideline on when to use which?