Random bad asserts for stat ops when running tests.

[dalejung]

======================================================================
FAIL: test_sum (pandas.tests.test_frame.TestDataFrame)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.6_with_system_site_packages/lib/python2.6/site-packages/pandas/tests/test_frame.py", line 10590, in test_sum
    has_numeric_only=True, check_dtype=False, check_less_precise=True)
  File "/home/travis/virtualenv/python2.6_with_system_site_packages/lib/python2.6/site-packages/pandas/tests/test_frame.py", line 10780, in _check_stat_op
    check_less_precise=check_less_precise)  # HACK: win32
  File "/home/travis/virtualenv/python2.6_with_system_site_packages/lib/python2.6/site-packages/pandas/util/testing.py", line 513, in assert_series_equal
    assert_almost_equal(left.values, right.values, check_less_precise)
  File "testing.pyx", line 58, in pandas._testing.assert_almost_equal (pandas/src/testing.c:2554)
  File "testing.pyx", line 93, in pandas._testing.assert_almost_equal (pandas/src/testing.c:1796)
  File "testing.pyx", line 140, in pandas._testing.assert_almost_equal (pandas/src/testing.c:2387)
AssertionError: expected 0.00144 but got 0.00144

[dalejung]

@jreback
It looks like the issue is that nanops specify the dtype_max when calling the ops. ff7bb2c seems to be the culprit.

In the case that I found, the unit test is checking a float32 frame vs that same frame up-casted to float64.

http://nbviewer.ipython.org/gist/anonymous/11349526

[jreback]

it could be but this error has been around a while actually

the issue is that np.sum is used as the comparison which should be passing .sum(dtype='float32') in this case

(the actual pandas routines are correct) after the fix above

[jreback]

pls submit a PR for this if you can (you can pass lambda x: np.sum(dtype='float32') instead of np.sum I think. This is sort a 'numpy' issue, really as np.sum is really doing the wrong thing

[jreback]

@dalejung I put up #6985, I *think * this should fix....can you reproduce reliably?

[jreback]

@dalejung if you notice any more pls lmk

[jreback]

@dalejung not sure my fix actually fixed this....!

[dalejung]

@jreback Hey, the last PR fixes this reliably. Just to clarify, if I have an array of float32, using a float64 accumulator is the correct behavior?

[jreback]

you can get away with the default accumulator on 64-bit systems because the default actually IS float64; however on 32-bit it breaks, but it will STILL work as long as it doesn't overflow.

so you need an overflow on 32-bit to fail, BUT using a 64-bit accumular is always safe, I think (and that's what I did)

[dalejung]

hm, I wasn't even thinking about overflow :/. I was more concerned about the output being different based on the accumulator. Not sure which output is technical correct. Like, the float64 is obviously more correct but I wasn't sure if there was an expectation of using float32 throughout the process.

[jreback]

I think it should be the same (though precision could affect it), so they could be slightly differently if accumulating really small numbers (that barely fit in float32). I would just always use float64, unless you have a really good reason.