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Re: Decimals and other numbers

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  Re: Decimals and other numbers Dave Angel <davea@davea.name> - 2015-01-08 21:43 -0500
    Re: Decimals and other numbers Marko Rauhamaa <marko@pacujo.net> - 2015-01-09 08:45 +0200

#83395 — Re: Decimals and other numbers

On 01/08/2015 09:33 PM, Devin Jeanpierre wrote:
> I noticed some very PHP-ish behavior today:
>
>>>> import decimal
>>>> x = 0
>>>> y = float(x)
>>>> z = decimal.Decimal(x)
>>>> x == y == z == x
> True
>>>> x ** x
> 1
>>>> y**y
> 1.0
>>>> z**z
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "/usr/lib/python2.7/decimal.py", line 2216, in __pow__
>      return context._raise_error(InvalidOperation, '0 ** 0')
>    File "/usr/lib/python2.7/decimal.py", line 3872, in _raise_error
>      raise error(explanation)
> decimal.InvalidOperation: 0 ** 0
>
>
> I'd file a bug report but I'm anticipating some rational (heh)
> explanation. Any ideas?
>
> Python 3.4 also raises this exception, but the error message is less
> informative. ("decimal.InvalidOperation: [<class
> 'decimal.InvalidOperation'>]").


What you don't say is which behavior you actually expected.  Since 0**0 
is undefined mathematically, I'd expect either an exception or a NAN result.


-- 
DaveA

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#83404

Dave Angel <davea@davea.name>:

> What you don't say is which behavior you actually expected. Since 0**0
> is undefined mathematically, I'd expect either an exception or a NAN
> result.

IEEE 754 mandates that 0**0 should evaluate to 1:

   <URL: http://en.wikipedia.org/wiki/NaN#Operations_generating_NaN>

    The standard pow function and the integer exponent pown function
    define 0**0, 1**∞, and ∞**0 as 1.

    The powr function defines all three indeterminate forms as invalid
    operations and so returns NaN.

Should IEEE 754's anomalies spill over to decimal, is a different
question.


Marko

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