精度问题，你用 decimal 从新计算
官方的一段文字，以前记的，忘了出处 Is math broken in Python? What’s the deal with the answer of 3220.0000000000005? This is an artifact of the floating point hardware on your computer only being able to accurately represent decimals in Base-2, not Base-10. For even simple calculations involving base-10 decimals, small errors are introduced. This is normal, although perhaps a bit surprising if you haven’t seen it before.