import string


def is_valid_letter(letter):
    """
    Returns True if the input letter contains only one character
    in ascii_letters. Both upper and lower case is OK. Otherwise
    returns False.
    :param letter: string
    :return: True or False
    """
    if len(letter) != 1:
        return False
    if letter not in string.ascii_letters:
        return False
    return True