The differences of _single_starting_underscore, __double_starting_underscores, __double_underscores__
1.DefineamethodorparameterwithanunderscoreasthestartingIfamethodstartswithanunderscore,itonlycanbecalledbythisclassitselforsubclass.Ofcourse,itcannotbeimportedbyusing"frommoduleimport*". 2.Defineameth