ref: https://www.datacamp.com/community/blog/python-pandas-cheat-sheet ref: https://pandas.pydata.org/Pandas_Cheat_Sheet.pdf ref: https://www.dataquest.io/blog/large_files/pandas-cheat-sheet.pdf ref: https://github.com/pandas-dev/pandas/blob/master/doc/cheatsheet/Pandas_Cheat_Sheet.pdf
