[New-bugs-announce] [issue27953] tan has poor accuracy near pi/2 on OS X Tiger

[Mark Dickinson]

New submission from Mark Dickinson:

Opening this for the record; I intend to close as "wont fix".

The tan function has poor accuracy for inputs near pi/2 on OS X 10.4. This is a direct consequence of a poor libm implementation, so there's little we can do about it short of re-implenting tan ourselves. 10.4 is ancient enough by now that it's difficult to care too much.

Example failures:

======================================================================
FAIL: test_testfile (test.test_math.MathTests)
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Traceback (most recent call last):
  File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_math.py", line 1190, in test_testfile
    '\n  '.join(failures))
AssertionError: Failures in test_testfile:
  tan0064: tan(1.5707963267948961): expected 1978937966095219.0, got 1978945885716843.0 (error = 7.92e+09 (31678486496 ulps); permitted error = 0 or 5 ulps)


and a corresponding failure in cmath:

======================================================================
FAIL: test_specific_values (test.test_cmath.CMathTests)
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Traceback (most recent call last):
  File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_cmath.py", line 398, in test_specific_values
    msg=error_message)
  File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_cmath.py", line 147, in rAssertAlmostEqual
    '{!r} and {!r} are not sufficiently close'.format(a, b))
AssertionError: tan0064: tan(complex(1.5707963267948961, 0.0))
Expected: complex(1978937966095219.0, 0.0)
Received: complex(1978945885716843.0, 0.0)
Received value insufficiently close to expected value.

----------
assignee: mark.dickinson
components: Extension Modules, Macintosh
messages: 274350
nosy: mark.dickinson, ned.deily, ronaldoussoren
priority: normal
severity: normal
status: open
title: tan has poor accuracy near pi/2 on OS X Tiger
type: behavior
versions: Python 2.7, Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27953>
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