Converts the given parse mode into an object with
    ``parse`` and ``unparse`` callable properties.

def sanitize_parse_mode(mode):
    """
    Converts the given parse mode into an object with
    ``parse`` and ``unparse`` callable properties.
    """
    if not mode:
        return None

    if callable(mode):
        class CustomMode:
            @staticmethod
            def unparse(text, entities):
                raise NotImplementedError

        CustomMode.parse = mode
        return CustomMode
    elif (all(hasattr(mode, x) for x in ('parse', 'unparse'))
          and all(callable(x) for x in (mode.parse, mode.unparse))):
        return mode
    elif isinstance(mode, str):
        try:
            return {
                'md': markdown,
                'markdown': markdown,
                'htm': html,
                'html': html
            }[mode.lower()]
        except KeyError:
            raise ValueError('Unknown parse mode {}'.format(mode))
    else:
        raise TypeError('Invalid parse mode type {}'.format(mode))