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

[Paul Moore]

On 8 March 2014 18:55, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Sat, 8 Mar 2014 18:49:02 +0000
> Mark Dickinson <dickinsm at gmail.com> wrote:
>>
>> If the proposal goes forward, I'll live with it, and will simply avoid
>> using the `Decimal` type to convert from floats or ints.  But I'd really
>> prefer to keep the short Decimal(x) spelling as something simple and
>> non-magic, and find more complicated ways (quotes!) of spelling the magic
>> stuff.
>
> For the record, as a non-float expert, Mark's arguments convince me.
>
> Just my 2 cents ;-)

I am in the same position. I "instinctively" wanted Decimal(1.1) to
return the same value as Decimal('1.1'), but as I read Mark's
argument, it became clearer to me that doing so involves a deliberate
loss of precision that is not present anywhere else in the chain of
conversions.

literal 1.1 -> float 1.1 is a precise conversion, in the sense that no
other float better represents the (decimal) literal 1.1.
The current float 1.1 -> decimal conversion is exact.

Converting float 1.1 to Decimal('1.1') deliberately drops precision,
even though an exact conversion is possible. I don't think that this
is something that should happen implicitly.

On the other hand, the *reason* people do Decimal(1.1) is because they
want a decimal value of 1.1. (Doh.) The fact that this isn't what they
are actually calculating is a mistake they make because Python doesn't
make it easy to *get* the decimal value 1.1 (I know the correct
approach is Decimal('1.1'), but I don't think that is obvious or
intuitive[1]). Decimal literals (1.1d) would resolve this issue
without introducing implicit rounding operations.

Paul

[1] The reason Decimal('1.1') feels non-obvious to me is that it feels
like it's getting to a decimal "via" a string. Of course, Decimal(1.1)
is also a going "via" a float. The *value* of the current behaviour is
that it leads people to an understanding that this is what it is.