Exercise for reference: 

Create a program that asks the user to input values separated by commas and stores those values in a text file, each value in a separate line.

Answer: 

line = input("Enter values: ")
line_list = line.split(",")
with open("user_data_commas.txt", "a+") as file:
    for i in line_list:
        file.write(i + "\n")

Explanation:

In line 1, we ask the user to input the values, and in line 2, we create a list containing the user submitted values. Then, in line 3, we open a file in a+  mode (read and append). If there's no file with such a name, it will be created. Then in lines 4 and 5, we iterate through the list and write those lines in the opened file.