Why does python break IEEE 754 for 1.0/0.0 and 0.0/0.0?

[Grant Edwards]

On 2005-07-14, Tim Peters <tim.peters at gmail.com> wrote:

> Python does go out of its way to raise ZeroDivisionError when
> dividing by 0.
>
>> 1/0 is defined by the standard as +Inf and 0/0 is NaN.
>>
>> That's what my platform does for programs written in C.
>
> IOW, that's what your platform C does (the behavior of these
> cases is left undefined by the C89 standard, so it's not the
> case that you can write a _portable_ C89 program relying on
> these outcomes).

True, but as a paracial matter, all of the C platforms I care
about all do obey IEEE 754.

> What does your platform C return for the integer expression
> 42/0?  Is any other outcome "wrong"?

I guess I though it was obvious from my reference to IEEE 754
that I was referring to floating point operations.  I don't
know (or generally care) what my C platform does for integer
divide by zero.

>> Python apparently checks for division by zero and throws and
>> exception rather than returning the correct value calculated
>> by the underlying platform.
>>
>> Is there any way to get Python to return the correct results
>> for those operations rather than raising an exception?
>
> No, except when using the decimal module.  The latter provides
> all the facilities in IBM's proposed standard for decimal
> floating-point, which intends to be a superset of IEEE 854:
>
>     http://www2.hursley.ibm.com/decimal/
>
> It's relatively easy to do this in the decimal module because
> it emulates, in software, all the gimmicks that most modern
> FPUs provide in hardware.
>
> Note that support for 754 was rare on Python platforms at the
> time Python was designed, and nobody mentioned 754 support as
> even a vague desire in those days.

I often foget how old Python is.  Still, I've been using IEEE
floating point in C programs (and depending on the proper
production and handling of infinities and NaNs) for more than
20 years now.  I had thought that Python might have caught up.

> In the absence of user interest, and in the absence of HW
> support for NaNs or infinities on most Python platforms,

Really?  I would have guessed that most Python platforms are
'586 or better IA32 machines running either Windows or Linux.
They all have HW support for NaNs and Infinities.

> the decision to raise an exception was quite sensible at the
> time. Python could not have implemented 754 semantics without
> doing emulating fp arithmetic in SW on most platforms (as the
> decimal module does today), and for much the same reasons you
> can't give a non-silly answer to my earlier "what does your
> platform C return for the integer expression 42/0?" question
> today <wink>.
>
>> There's no way to "resume" from the exception and return a
>> value from an exception handler, right?
>
> Correct.
>
> Note that there's a huge, current, informed discussion of
> these issues already in the math.nroot thread.

-- 
Grant Edwards                   grante             Yow!  It's OKAY --- I'm an
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