round()函数计算四舍五入后有时也没有按照要求的位数保存啊
a=3.14
b=1.57
s=a*b
print(s) # ==>4.9298
print(round(s,0)) # ==>5.0
print(round(s,1)) # ==>4.9
print(round(s,2)) # ==>4.93
print(round(s,3)) # ==>4.93
print(round(s,4)) # ==>4.9298
print(round(s,5)) # ==>4.9298
print(round(3.2,0)) # ==>3.0
print(round(3.2,1)) # ==>3.2
