docs: def format_field(self, value, format_spec):
          """Format specifiers are described in :func:`format_field` which is a
          static function.
          """
          if format_spec:
              spec, arg = format_spec[0], format_spec[1:]
              arg = arg or None
          else:
              spec = arg = None
          return self._format_field(spec, arg, value, self.numeric_locale)