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

[David Mertz]

On Sat, Mar 8, 2014 at 10:49 AM, Mark Dickinson <dickinsm at gmail.com> wrote:

> Accepted, but I believe the proposal would break a number of other
> expectations too with respect to ints and floats.  For one, we currently
> have the unsurprising, and I believe desirable, property that conversion to
> Decimal is monotonic: for any finite numbers (int, float or Decimal) x and
> y,  if x <= y then Decimal(x) <= Decimal(y).  The proposal would break that
> property, too: you could find examples of an integer x and float y such
> that `x < y` and `Decimal(x) > Decimal(y)` would be simultaneously True.
>

I think that Mark Dickinson's point about breaking monotonicity is a STRONG
argument against the proposal to change the meaning of Decimal(float_num)
to produce Decimal(repr(float_num)).  That latter spelling is already
available if anyone wants it.

The focus on float value that have been entered as literals feels like a
distraction to me.  Floats come from other places as well, and making
Decimal(float_num) produce the *exact* value feels far more desirable than
producing "something which rounds to the same float.

As I think Guido has acknowledged in a recent post, a far better and more
intuitive approach is just to make decimal literals easier to write.
 Teaching users to write '3.14d' doesn't feel that hard to me, and it looks
notably prettier than 'decimal.Decimal("3.14")'.  A *would* also like an
optional explicit binary-float literal though, e.g. '3.14f'.  This wouldn't
actually mean anything different from '3.14', but then "PI" doesn't mean
anything different from u"PI" either.  The optional bin-float specifier, in
principle, allows for some Python 4000 in which decimal is the default
literal (although I don't think I'd ever support that, but no harm in
letting that decision be made years from now).

- if we're aiming to eliminate surprises, the 'fix' doesn't go far enough:
> Decimal(1.1 + 2.2) will still surprise, as will
> Decimal(0.12345123451234512345)
>

This is a simple illustration of why the "do what I mean" goal of the
proposed change will fall apart rather quickly for anything but the
simplest examples.  The status quo of Decimal(float_num) producing the
*exact* value continues to feel the best to me.

I've said too much already;  beyond registering my strong -1 on this
> proposal, I'm going to keep out of further discussion.
>

So yeah, me too: -1 on proposal.



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