defaultfilters test passes unexpectedly under Python 3

[Russell Keith-Magee]

defaultfilters.DefaultFiltersTests.test_floatformat_fail is currently marked as an expected failure; and under Python 2, it does fail.

Under Python 3 (at least, with Python 3.2.3 64 bit under OSX), the test passes.

It's entirely possible that the resolution is "don't worry about it" - after all, there are worse problems than an unexpected test pass. However, since it is an oddity in test suite results that may have gone unnoticed (since 'unexpected pass' would be easy to miss in test suite results), I'm marking this as a release blocker.

[Aymeric Augustin]

The test that unexpectedly passes is:

self.assertEqual(floatformat(1.00000000000000015, 16), '1.0000000000000002')

Under Python 2, it returns '1.0000000000000000'.

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The explanation is pretty simple.

The first step of the floatformat template filter is to coerce its input as text, and this step introduces the difference between Python 2 and Python 3.

% python2.6
>>> 1.00000000000000015
1.0000000000000002
>>> str(_)
'1.0'

% python3.2
>>> 1.00000000000000015
1.0000000000000002
>>> str(_)
'1.0000000000000002'

I think it's related to this paragraph of ​Python 3.1's release notes:

Python now uses David Gay’s algorithm for finding the shortest floating point representation that doesn’t change its value.

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Arguably, this is a bug in Python 2 that was fixed in Python 3. I'm going to mark the test as an expected failure only on Python 2.

[Aymeric Augustin <aymeric.augustin@…>]

In 8d7e526229c27567860f7479dbce15d245742feb:

[1.5.x] Fixed #18964 -- floatformat test passes under py3k

Thanks Russell for the report.

Backport of b4420d9 from master.