code: def _compose_style(self):
          
          valid_styles = (
              'color', 'width', 'filter', 'strokeWidth', 'strokeColor',
          )
          self.styling = '\n'.join(
              '{prop}: {style}'.format(prop=s, style=getattr(self, s))
              for s in valid_styles
              if getattr(self, s) is not None
          )
docs: Appends `prop` with `style` to layer styling