为什么 Decimal modulo 给出错误的符号？

Question

花了很长时间来追踪一个错误，这个错误基本上归结为我假设以下两个模都会 return 结果大于 0:

>>> -0.09 % 0.1
0.010000000000000009  # OK

>>> Decimal('-0.09') % Decimal('0.1')
Decimal('-0.09')      # Not OK -- I expected a positive value!

这种行为似乎违背了Python docs:

The modulo operator always yields a result with the same sign as its second operand (or zero)

这是一个错误，还是我做错了什么？

Answer 1

您正在查看错误的文档：您链接的段落适用于内置数字类型（尽管这一点并不明显）。对于 decimal，您需要查看 documentation of decimal。上面写着：

remainder(x, y)

Returns the remainder from integer division.

The sign of the result, if non-zero, is the same as that of the original dividend.

此外：

The % and // operators implement the remainder and divide-integer operations (respectively) as described in the specification.

这种行为不一致的事实当然很烦人。

Answer 2

这不是错误。这是 Decimal 在 Python 中的（不幸的）工作方式，它与整数或浮点数不一致：

There are some small differences between arithmetic on Decimal objects and arithmetic on integers and floats. When the remainder operator % is applied to Decimal objects, the sign of the result is the sign of the dividend rather than the sign of the divisor:

来源：https://docs.python.org/2/library/decimal.html

为什么 Decimal modulo 给出错误的符号？

Why does Decimal modulo give the wrong sign?

python

math

modulo