[Python-Dev] math.sqrt(-1) - NaN, ValueError or whatever you want? (original) (raw)

Tim Peters tim.one at comcast.net
Thu Feb 5 17:19:39 EST 2004

[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 .

How?

You'll understand the "" 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).

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