pythonunittest2assertAlmostEqual 与 `places` 工作不正确

Question

我正在使用 unittest2 处理以下问题:

assertAlmostEqual(69.88, 69.875, places=2)  # returns True

但是

assertAlmostEqual(1.28, 1.275, places=2)  # returns False

我认为问题出在 assertAlmostEqual 方法中:

def assertAlmostEqual(self, first, second, places=None, ...):
    if first == second:
        # shortcut
        return
    ...
    if delta is not None:
    ...
    else:
        if places is None:
            places = 7

        if round(abs(second-first), places) == 0:
            return

    ...
    raise self.failureException(msg)

应该是:

if abs(round(second, places) - round(first, places)) == 0
    return

Answer 1

您提出的修复方案不会产生任何影响，您可以轻松证明:

>>> places = 2
>>> first, second = 69.88, 69.875
>>> round(abs(second-first), places)
0.0
>>> abs(round(second, places) - round(first, places))
0.0

这是浮点精度的问题，请参见例如Is floating point math broken? 69.88 无法准确表示:

>>> "{:.40f}".format(69.88)
'69.8799999999999954525264911353588104248047'