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

[Guido van Rossum]

Any solutions you are going to come up with would cause other anomalies
such as -(5d) not being equal to -5d. My vote goes to treating + and - as
the operators they are and telling people that if they want a negative
constant that exceeds the context's precision they're going to have to use
Decimal('-N'), rather than adding a terrible hack to the parser (and hence
to the language spec).


BTW Do you why the default precision is 28?


On Sun, Mar 9, 2014 at 1:07 PM, Oscar Benjamin
<oscar.j.benjamin at gmail.com>wrote:

> On 9 March 2014 19:34, Guido van Rossum <guido at python.org> wrote:
> >
> > I hope that somebody will take up the task to write a PEP about
> introducing
> > a decimal literal; that sounds like an obtainable goal, less
> controversial
> > than changing Decimal(<float>).
>
> I said I'd have a go at this. It's not as simple as it might seem though.
>
> The simplest proposal would be to say that 3.14d is equivalent to
> Decimal('3.14'). The fact the the constructor is independent of the
> context is good here. It means that the value of the literal can be
> determined statically which is good for trying to understand what code
> does and also for constant folding etc.
>
> 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'):
>
>     >>> from decimal import Decimal, getcontext
>     >>> getcontext().prec = 3
>     >>> Decimal('+1234')  # Exact
>     Decimal('1234')
>     >>> +Decimal('1234')  # Rounded
>     Decimal('1.23E+3')
>     >>> Decimal('-1234')  # Exact
>     Decimal('-1234')
>     >>> -Decimal('1234')  # Rounded
>     Decimal('-1.23E+3')
>
> I think that this behaviour would be surprising from a literal. I
> would prefer to be able to say that -1234d was equivalent to
> Decimal('-1234d') and would be guaranteed not to round. I just don't
> really know if that's trivially easy or really difficult given the
> current grammar specification and parse/compiler infrastructure.
>
>
> Oscar
>



-- 
--Guido van Rossum (python.org/~guido)
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