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)
----------------------------------------------------------------------
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)
----------------------------------------------------------------------
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.

New changeset b4d52df5595e by Mark Dickinson in branch 'default':
Issue #27953: skip failing math and cmath tests for tan on OS X 10.4.
https://hg.python.org/cpython/rev/b4d52df5595e

The relevant tests are now skipped on OS X < 10.5, and test_math and test_cmath are passing on the OS X Tiger buildbot. Closing (as "wont fix", since we haven't actually fixed the underlying issue).

It would be nice to use the existing @requires_mac_ver() decorator to skip the tests (see Lib/test/support/__init__.py).

Ned: that's for skipping an entire test method, right? Here a single test method tests a few hundred different special cases; we want to exclude only one of those cases.

Mark, ah, yes, that's true.  So the tests would have to be separated out into a separate test case method.  So, not worth the effort.

Given enough time, it would make a lot of sense to adapt these tests to use the subtests machinery (though it's not immediately clear to me how to make @requires_mac_ver work with the subtests machinery). I'm afraid this issue was a bit of a quick fix to get the buildbots back to working state.