Help with function and sep( , ) value

[GOTO10]

How are you calling your function(s) when you receive the error? As is, your code appears to work correctly when given a list of strings as input:

Output:
>>> mystringlist = ['a','b','c','d']
>>> to_string(mystringlist)
'a, b, c, d'

It throws type errors for a list of anything other than strings, but that could be fixed by using the str() function when building your result in to_string(). (That appears to be within the rules the way I am reading them, as it only mentions not using built-in functions for the length() part of the assignment.)