最近在项目中遇到前后端计算数值不一样的情况，发现是由于 Python3 数学运算处理方式和 PHP BC Math 不一样
PHP 的 round 函数
round ( float $val , int $precision = 0 , int $mode = PHP_ROUND_HALF_UP ) : float
| Constants | Description |
|---|---|
| PHP_ROUND_HALF_UP | Rounds num away from zero when it is half way there, making 1.5 into 2 and -1.5 into -2. |
| PHP_ROUND_HALF_DOWN | Rounds num towards zero when it is half way there,making 1.5 into 1 and -1.5 into -1. |
| PHP_ROUND_HALF_EVEN | Rounds num towards the nearest even value when it is half way there, making both 1.5 and 2.5 into 2. |
| PHP_ROUND_HALF_ODD | Rounds num towards the nearest odd value when it is half way there, making 1.5 into 1 and 2.5 into 3. |
其中 PHP_ROUND_HALF_UP 是默认的
Python3 的 round 函数没有处理浮点数运算的问题
round(number[, ndigits])
The behavior of [round()] for floats can be surprising
https://docs.python.org/3/library/functions.html?highlight=round#round
round(2.675, 2) gives 2.67
而 decimal 可以四舍五入等运算方式，不过需要指定参数 rounding
可选值为
decimal.ROUND_CEILING
Round towards Infinity.
decimal.ROUND_DOWN
Round towards zero.
decimal.ROUND_FLOOR
Round towards -Infinity.
decimal.ROUND_HALF_DOWN
Round to nearest with ties going towards zero.
decimal.ROUND_HALF_EVEN
Round to nearest with ties going to nearest even integer.
decimal.ROUND_HALF_UP
Round to nearest with ties going away from zero.
decimal.ROUND_UP
Round away from zero.
decimal.ROUND_05UP
Round away from zero if last digit after rounding towards zero would have been 0 or 5; otherwise round towards zero.
用法如下
>>> from decimal import Decimal
>>> Decimal('2.145').quantize(Decimal("0.00"), rounding = "ROUND_HALF_UP")
Decimal('2.15')
也可以用 context
>>> import decimal
>>> decimal.getcontext().rounding = "ROUND_HALF_UP"
>>> decimal.Decimal('2.145').quantize(decimal.Decimal("0.00"))
Decimal('2.15')
即可实现一般财务计算中四舍五入的方式
>>> decimal.Decimal('2.675').quantize(decimal.Decimal("0.00"))
Decimal('2.68')