[Python-ideas] Changing Decimal.__pos__ and Decimal.__neg__ [Was: Re: Python Numbers as Human Concept Decimal System]

[Oscar Benjamin]

On 10 March 2014 08:32, Mark Dickinson <dickinsm at gmail.com> wrote:
> On Sun, Mar 9, 2014 at 8:07 PM, Oscar Benjamin <oscar.j.benjamin at gmail.com>
> wrote:
>>
>> The problem though is with things like +3.14d or -3.14d. Python the
>> language treats the + and - signs as not being part of the literal but
>> as separate unary operators. This is unimportant for int, float and
>> imaginary literals because there unary + is a no-op and unary - is
>> exact. For decimal this is not the same as +Decimal('3.14') is not the
>> same as Decimal('+3.14'):
>
> Another option would be to change the behaviour of + and - on Decimal
> objects by binding `__pos__` and `__neg__` to the specification's 'copy' and
> 'copy-negate' operations instead of (as currently) to the 'plus' and 'minus'
> operations.  Those behaviours are more in keeping with the way that + and -
> operate on other objects.

Hi Mark and thanks for responding,

Do you think it's really feasible to make such a change? The use of
unary + for rounding to context has been a feature of the decimal
module for as long as I've been using it. It's shown in the docs both
in the Recipes section and the FAQ.

I just advised Mark Harris to do this and it know appears ~20 times in
his pdeclib module:
https://code.google.com/p/pythondecimallibrary/source/browse/pdeclib.py#207

I would expect that a change such as this would break quite a lot of
code (some of mine would break) so there would have to be some kind of
compatibility mechanism or deprecation process or something but I
can't think of a good way of doing it.


Oscar