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Re: [Python-Dev] == on object tests identity in 3.x

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  Re: [Python-Dev] == on object tests identity in 3.x Ian Kelly <ian.g.kelly@gmail.com> - 2014-07-08 16:05 -0600
    Re: [Python-Dev] == on object tests identity in 3.x Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-09 01:04 +0000

#74214 — Re: [Python-Dev] == on object tests identity in 3.x

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.

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#74221

On Tue, 08 Jul 2014 16:05:45 -0600, Ian Kelly wrote:

> 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.

Correct:


py> numpy.log(-3)
nan
py> if numpy.log(-3) == numpy.log(-5):
...     print "-3 == -5"
...
py> 


-- 
Steven

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