Override :meth:`string.Formatter.format_field` to have our
         default format_spec for :class:`datetime.Datetime` objects, and to
         let None yield an empty string rather than ``None``.

def format_field(self, value, format_spec):
        """Override :meth:`string.Formatter.format_field` to have our
         default format_spec for :class:`datetime.Datetime` objects, and to
         let None yield an empty string rather than ``None``."""
        if isinstance(value, datetime) and not format_spec:
            return super().format_field(value, '%Y-%m-%d_%H-%M-%S')
        if value is None:
            return ''
        return super().format_field(value, format_spec)