import string


def check_guess(player_guess, guesses):
    """(string, [string]) -> string

    Check if the guess provided by the user is a correct guess -
    that is, one that is just 1 letter and not guessed before.

    >>> check_guess('', [])
    'Please enter at least one letter.'
    >>> check_guess('bacon', [])
    'Please enter only one character.'
    >>> check_guess('c', ['f', 'c', 'g', 'e'])
    'Please pick a letter you did not guess before.'
    >>> check_guess('bacon', ['c', 'a', 'b', 'o', 'n'])
    'Please enter only one character.'
    """
    message = ""
    if len(player_guess) == 1:
        if player_guess not in string.ascii_letters:
            message = "Please enter only letters."
        elif player_guess.lower() in guesses:
            message = "Please pick a letter you did not guess before."
    elif len(player_guess) == 0:
        message = "Please enter at least one letter."
    else:
        message = "Please enter only one character."
    return message