保留浮点数的小数点。

     如保留小数点后两位。

     num = 9.2174

     new_num = round( num , 2 )

     则new_num = 9.22    (四舍五入）

>>> num = 4.93
>>> num
4.93
>>> new_num = round(num,0)
>>> new_num
5.0
>>> num = 4.49
>>> new_num = round(num,0)
>>> new_num
4.0
>>> new_num = round(num,0)