def __str__(self):
        """A human-readable string representation (truncated if it gets too
        long).

        """

        def format_item(item):
            return "{!s}: {!s}".format(*item)

        MAX_LENGTH = 20
        half = MAX_LENGTH // 2
        if len(self) > MAX_LENGTH:
            one = ", ".join(format_item(_) for _ in self._d.items()[:half])
            two = ", ".join(format_item(_) for _ in self._d.items()[half:-half])
            three = ", ".join(format_item(_) for _ in self._d.items()[-half:])
            truncate_string = f"<...{len(self) - MAX_LENGTH} items...>"
            if len(truncate_string) < len(two):
                two = truncate_string
            items = ", ".join([one, two, three])
        else:
            items = ", ".join(format_item(_) for _ in self._d.items())
        default = f", default={self.default!r}".format(self.default) if self.default else ""
        return f"{type(self).__name__}({{{items}}}{default})"