How can I improve my commacode?

Question

def list_value(*spam, sep=','):
li =list(*spam)
return sep.join(li)
li=list_value(input())
l='{} and {}'.format((li[:-1),(li[-1]))
print(list_value(l))

just help me fix my code, so I could have the expected result.
I have no idea what mess I have created, that the output is so obscure.

if input: ['apple', 'banana', 'mango']
expected output: 'apple, banana, and mango'

Answer 1

def deco (list):
res = ""
[(res = res + el + ", ") for el in list[:-1]]
return res + "and " + str (list[-1])