code: def format_field(self, value, format_spec):
          
          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)
docs: 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``.