On Wed, Oct 14, 2020 at 03:33:22PM -0400, Random832 wrote:
> That is nonsense. "exactly representable" is a plain english phrase
> and has a clear meaning that only involves the actual data format, not
> the context.
Perhaps your understanding of plain English is radically different from
mine, but I don't understand how that can be.
The actual data format has some humongeous limits (which may or may not
be reachable in practice, due to memory constraints). It is obvious to
me that "exactly representable" must take into account the current
context:
- If it didn't, then the context precision would be meaningless;
changing it wouldn't actually change the precision of calculations.
- Decimal(1)/3 is Decimal('0.3333333333333333333333333333') by default,
a mere 28 digits, not MAX_PREC (999999999999999999) digits.
> Nonsense. The meaning of the inexact flag means that the module
> *chose* to discard some information.
Is it truly a choice, if the module has no choice? Do you expect the
decimal module to return an exact result for 1/3 or sqrt(2)?
I think that you are just surprised that decimal rounds operations to
the current context, not to the implied precision of the operands. My
guess is that you were expecting arithmetic on operands to honour their
precision:
getcontext().prec = 3
Decimal('0.111111111111') + Decimal('0.222222')
# expect Decimal('0.333333111111')
# but get Decimal('0.333')
but (aside from "wishful thinking") I can't see why you expected that.
(To be honest, I was surprised to learn that the context precision is
ignored when creating a Decimal from a string. I still find it a bit odd
that if I set the precision to 3, I can still create a Decimal with
twelve digits.)
--
Steve
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