def username_val(cursor, username):
    """Check to see if the requested username already exists.

    Args:
      cursor:
        A GqlQuery object, querying the User database and including
          all columns.
          Docs for the GqlQuery class can be found here:
          https://cloud.google.com/appengine/docs/standard/python/datastore/gqlqueryclass
      username:
        Any valid username (doesn't violate the USER_RE regex
          and is not a number.)

    Returns:
      A Boolean representing the validity of the username: False if it
      already used, True if it is not yet in the database.
    """
    for each in cursor:
        if each.name == username:
            return False
    return True