python unittest2 assertAlmostEqual with `places` works incorrectly

Question

I am dealing with the following problem with unittest2:

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

but

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

I think problem is in the assertAlmostEqual method:

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)

Should it instead be:

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

score 2 · Accepted Answer · edited May 23 '17 at 12:06

Your proposed fix doesn't make any difference, as you can easily demonstrate:

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

This is a problem with floating point precision, see e.g. Is floating point math broken? 69.88 cannot be represented exactly:

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

score 0 · Answer 2 · answered Feb 24 '15 at 13:21

The difference in the second example is

0.005

And even without mentioned biases of floating points result of round will be 0.01, so these numbers really different with 2-places precision

This method compares difference between numbers. It is kinda standard of comparing float numbers actually

So the problem is not with implementation, but with you expectations, that is different from common float comparison

python unittest2 assertAlmostEqual with `places` works incorrectly

2 Answers2