You can use the str.split method.
>>> my_string = 'A,B,C,D,E'
>>> my_list = my_string.split(",")
>>> print my_list
['A', 'B', 'C', 'D', 'E']
If you want to convert it to a tuple, just
>>> print tuple(my_list)
('A', 'B', 'C', 'D', 'E')
If you are looking to append to a list, try this:
>>> my_list.append('F')
>>> print my_list
['A', 'B', 'C', 'D', 'E', 'F']