[SciPy-Dev] Platform-dependent tests?

[Warren Weckesser]

Jeff Armstrong has enhanced the Schur decomposition function
scipy.linalg.schur to expose the ability to sort the eigenvalues (more
accurately, group them according to a boolean function); see
https://github.com/scipy/scipy/pull/23.  schur() is a wrapper for the DGEES
Fortran library function.  According to its documentation, if the matrix is
poorly scaled, the eigenvalues can fail to satisfy the sorting condition
*after* they've been sorted, because of rounding errors.  The function
returns an error code when this occurs.

Jeff's code checks for this condition, and he also added a couple unit tests
for it.  Unfortunately, the tests fail on my computer--that is, the error
condition does not happen.  This is not surprising, as the error condition
relies on the behavior of rounding errors.  So, to the question:  is there a
way to make these tests platform-dependent, so that they only run on a
platform/architecture/whatever where the tests are known to trigger the
desired condition?  Or is the result likely to depend on so many other
factors (compiler and optimization settings, third party math library, phase
of the moon, etc) that it is hopeless to try?

Warren
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