On OS X 10.4, and probably other versions of OS X too, calls to math.log and math.sqrt that 
should raise ValueError instead return a NaN:

Python 2.6a0 (trunk:60144, Jan 20 2008, 21:43:56) 
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from math import sqrt, log 
>>> sqrt(-1)
nan
>>> log(-1)
nan

The problem is that OS X doesn't set errno in these cases.  Attached is a quick fix for this.  
Note that there's already a test for this (test_exceptions in test_math.py), but this test is 
only run in verbose mode.

See also issue #871657.