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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Tue, 8 Jul 2014 16:05:45 -0600
Subject: Re: [Python-Dev] == on object tests identity in 3.x
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On Tue, Jul 8, 2014 at 3:53 PM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On Tue, Jul 8, 2014 at 2:10 PM, Anders J. Munch <2014@jmunch.dk> wrote:
>> Steven D'Aprano wrote:
>>> - Keeping reflexivity for NANs would have implied some pretty nasty
>>>    things, e.g. if log(-3) == log(-5), then -3 == -5.
>>
>>
>>>>> log(-3)
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in <module>
>> ValueError: math domain error
>>
>> You were perhaps referring to the log functions in C and Fortran, not
>> math.log?
>> The tradeoffs are different in those languages, so choices the IEEE-754
>> committee made with C and Fortran in mind may be less relevant for Python.
>
>>>> import ctypes
>>>> libm = ctypes.cdll.LoadLibrary("libm.so.6")
>>>> log = libm.log
>>>> log.argtypes = [ctypes.c_double]
>>>> log.restype = ctypes.c_double
>>>> log(-3)
> nan
>>>> log(-5)
> nan
>>>> log(-3) == log(-5)
> False

Also, numpy provides more control over floating-point error handling
than straight Python does, and I think (but can't presently test) that
numpy.log(-3) will return nan by default.