Some functions to query users at the command prompt.

query.py

def general_query(question, validator, default=None):
    """
    Asks the given question, returning the (possibly modified) result if validator
    allows it, and keeps asking until a valid answer is given.

    :param str question: The question to ask.
    :param validator: A function that acts on user response strings, and returns
        `(success, modified_response, message)`. The loop ends if `success` is `True`,
        in which case `modified_response` is returned. `message` is printed in
        any case.
    :param default: The implied response when the user provides a zero-length
        response (just presses Enter).
    """
    if default is not None and not validator(default)[0]:
        raise ValueError("Default value not valid...")
    while True:
        print(question)
        response = input()
        response = default if default is not None and response == "" else response
        success, value, msg = validator(response)
        if len(msg) > 0:
            print(msg)
        if success:
            return value


def folder_query(question, default_folder=None):
    """
    Queries the user for a valid folder.

    :param str question: The question to ask, where the `format` is invoked with
        `default_folder` as the argument.
    :param str default_folder: The default folder to use. If `None`, the current
        working directory is used.
    :return: The folder.
    :rtype: `str`
    """

    def validator(folder):
        success = os.path.isdir(folder)
        msg = "" if success else "Specified folder does not exist."
        return success, folder, msg

    if default_folder is None:
        default_folder = os.path.realpath(os.getcwd())

    question = question.format(default_folder)
    return general_query(question, validator, default=default_folder)


def query_yes_no(question, default=None):
    """
    Queries the user for a yes/no response.

    :param str question: The question to ask.
    :param default: The default user response, 'y', 'n', or `None` for no default.
    :return: `True` if 'yes', `False` if `no`
    :rtype: `bool`
    """
    valid = {"yes": True, "y": True, "ye": True, "no": False, "n": False}

    prompt = (
        " [y/n] " if default is None else " [Y/n] " if valid[default] else " [y/N] "
    )

    def validator(response):
        success = response.lower() in valid
        result = valid[response] if success else None
        msg = "" if success else "Please respond with 'yes' or 'no' (or 'y' or 'n')"
        return success, result, msg

    return general_query("{} {} ".format(question, prompt), validator, default)