code: def _format_number_si(num, n_signif_figures):
      
      if math.isnan(num) or math.isinf(num):
          return str(num)
      leading, exp = '{:E}'.format(num).split('E')
      leading = round(float(leading), n_signif_figures - 1)
      exp = exp[:1] + exp[2:] if exp[1] == '0' else exp
      formatted = '{}e{}'.format(leading, exp.lstrip('+'))
      return formatted
docs: Format a number using scientific notation to given significant figures