def _check_username(username):
"""
Checks that a username is valid and is not already in use. Returns
the username if valid, or None if it is not.
"""
checked_name = None
if username is not None and username != "":
user_db = UserDB()
exists = user_db.find_user_by_name(username)
if exists == []:
checked_name = username
return checked_name
