c = [1, 2, 3, 4, 5] class TEST(): c = [5, 2, 3, 4, 5] def add( self ): c[0] = 15 a = TEST() a.add() print( c, a.c, TEST.c ) result : [15, 2, 3, 4, 5] [5, 2, 3, 4, 5] [5, 2, 3, 4, 5] why a.add() do not update c in Class TEST? but update c in main file