[Python-ideas] Python Numbers as Human Concept Decimal System

[Oscar Benjamin]

On 11 March 2014 15:14, Antoine Pitrou <solipsis at pitrou.net> wrote:
>
>>> It is arguably much simpler than unicode and character
>>> encodings. Besides, for most applications Decimal's current precision
>>> will
>>> be good enough: changing the current context (which, I believe, is
>>> thread-local, not process-global) is only necessary in special cases.
>>
>>
>> The problem is that the context cannot be depended upon so the result
>> of trivial calculations or even a negative decimal literal will depend
>> on something that can be changed *anywhere* else in the code. The
>> expression -1.123456789d will evaluate differently depending on the
>> context and I think many people will be surprised by that. I expect
>> that this would lead to naive code that breaks when the context is
>> changed. Great care needs to be taken to get this right, or otherwise
>> it should just be considered generally unsafe to change the context.
>
> This is a good point, but I still don't think it outweighs the large
> cognitive baggage (and maintenance overhead for library authors as well as
> ourselves) of having two different types doing mostly similar things.
>
> It is also not clear that a decimal type would be really beneficial for
> non-expert users. Most people get by with floats fine.

These are good arguments. I guess we'll just have to see what all the
pros and cons look like taken together. (There's always the
possibility of doing nothing of course!)

>> But let's take something similar: BigDecimal(1).divide(BigDecimal(3)).
>> This expression cannot be computed exactly in decimal format so it
>> raises ArithmeticError. If you want it to round to some precision then
>> you can provide a context with BigDecimal(1).divide(BigDecimal(3),
>> mymathcontext).
>
> But then it becomes, ironically, more cumbersome to spell than
>   Decimal(1) / Decimal(3)
> (I'm talking about the explicit context, of course, not the .divide() method
> which needn't exist in Python)

That's why I propose something more along the lines of what the other
languages have done.

I'm not sure if any other languages have the same model as Python's
decimal module except possibly REXX. I don't have any experience of
REXX but from a quick read of some docs the "numeric digits"
instruction seems to have the same global effect as the decimal
module's contexts.


Oscar