[Jack Jansen]
The answers to my math questions never seem to stick, so I'm afraid I'm gonna have to ask this question again (even though I think I did the same a couple of years ago, but google doesn't find it).
It turns out that on MacOS math.sqrt(-1) returns "nan", as does math.log(-1), and I didn't try any other ones.
This is true for all possible combinations of MacPython 2.3, 2.4a0 or apple-Python 2.3, MacPython-OS9 or MacPython-OSX, and Mac OS 9 or Mac OS X.
When I tried the same on a linux box (Python 2.4a0) it raised ValueError: math domain error, which is what I would have expected.
Take this to heart (it's from the current math module docs): Note: The math module consists mostly of thin wrappers around the platform C math library functions. Behavior in exceptional cases is loosely specified by the C standards, and Python inherits much of its math-function error-reporting behavior from the platform C implementation. As a result, the specific exceptions raised in error cases (and even whether some arguments are considered to be exceptional at all) are not defined in any useful cross-platform or cross-release way. For example, whether math.log(0) returns -Inf or raises ValueError or OverflowError isn't defined, and in cases where math.log(0) raises OverflowError, math.log(0L) may raise ValueError instead.
Why this difference?
Platform C.
Is it something I should fix?
Good luck <wink>.
How?
You'll understand the "<wink>" above after you give up.
Or document it?
I think the above already does. Else you're trying to document a maze of moving targets (including, on some platforms, the specific compiler options, link options, and environment variables the user indulges).