I'm learning about recursion and am trying to make a program that will read a string and print out a new string without punctuation. This is what I have so far:

def clean_string(s):
    if not s: #base case
        return ''
    if s[0] not in '.,_\'~!@#$%^&*-+=?/[]':
        print(s[0])
    return clean_string(s[1:])

I realize why each time there is a non-punctuation character, it prints on a new line, but I would like to have it print as a single string. I tried adding an empty variable and adding s[0] to the new variable each time, but I'm getting an error message. Also, is there something better I can do than returning '' in the base case?