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

[Steven D'Aprano]

On Mon, Mar 10, 2014 at 10:07:11AM +0000, Oscar Benjamin wrote:
> On 9 March 2014 20:39, Guido van Rossum <guido at python.org> wrote:
> > On Sun, Mar 9, 2014 at 1:07 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.

Is that still true? Possibly the peephole optimizer has changed the 
situation?

Using Python 1.5:

>>> from dis import dis
>>> dis(compile("-5", "", "single"))
          0 SET_LINENO          0

          3 SET_LINENO          1
          6 LOAD_CONST          0 (5)
          9 UNARY_NEGATIVE
         10 PRINT_EXPR
         11 LOAD_CONST          1 (None)
         14 RETURN_VALUE


but using Python 2.4:

py> dis(compile("-5", "", "single"))
  1           0 LOAD_CONST               0 (-5)
              3 PRINT_EXPR
              4 LOAD_CONST               1 (None)
              7 RETURN_VALUE

(versions more recent than 2.4 also use a constant).


> >> 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'):
> 
> > 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).
> 
> I think it's reasonable that -(5d) not be equal to -5d.

Did you leave the word "not" out of that sentence? :-)

I don't think that it is reasonable for -(5d) to not equal -5d. I think 
that's an awful interface, and terribly surprising. I don't think that 
there are any situations were an otherwise redundant pair of parens 
changes behavious. E.g. x*(y) is the same as x*y.

(Parens are sometimes used to disambiguate syntax or change precedence. 
That's different, since the parens aren't redundant.)

Considering that the motivation for this change is to make it easier for 
newbies and numerically naive users, I really do not relish having to 
explain to newbies why -5d and -(5d) are different.


-- 
Steven