44-line Python rot13 script

rot13.py

import string

charset = list(string.ascii_lowercase)

def rem_numbers(source):
    new_string = []
    to_remove = list(string.digits)
    source = list(source.lower())
    for char in source:
        if char not in to_remove:
            new_string.append(char)
    return("".join(new_string))

def crypt(source, rot, param):
    source_list = list(source)
    result_list = []
    for char in source_list:
        try:
            index = charset.index(char)
            if param is True:
                result_list.append(charset[(index + rot) % len(charset)])
            else:
                result_list.append(charset[(index - rot) % len(charset)])
        except:
            result_list.append(" ")
    return("".join(result_list))

def main():
    rot_by = int(input("Rotate by how many characters?:"))
    while True:
        rot_type = str(input("Encrypt or decrypt? (e/d):"))
        rot_type = rot_type[0].lower()
        if rot_type == "e" or rot_type == "d":
            break
    rot_string = rem_numbers(str(input("String to rot:")))
    result = None
    if rot_type == "e":
        result = crypt(rot_string, rot_by, True)
    else:
        result = crypt(rot_string, rot_by, False)
    print(result)

while True:
    main()