[Python-Dev] Mixing float and Decimal -- thread reboot

[Greg Ewing]

Raymond Hettinger wrote:
> The question of
> where to stack decimals in the hierarchy was erroneously 
> being steered by the concept that both decimal and binary floats
> are intrinsically inexact.  But that would be incorrect, inexactness
> is a taint, the numbers themselves are always exact.

I don't think that's correct. "Numbers are always exact" is
a simplification due to choosing not to attach an inexactness
flag to every value. Without such a flag, we don't really know
whether any given value is exact or not, we can only guess.

The reason for regarding certain types as "implicitly inexact"
is something like this: If you start with exact ints, and do
only int operations with them, you must end up with exact
ints. But the same is not true of float or Decimal: even if
you start with exact values, you can end up with inexact ones.

> I really like Guido's idea of a context flag to control whether
> mixing of decimal and binary floats will issue a warning.

Personally I feel that far too much stuff concerning decimals
is controlled by implicit context parameters. It gives me the
uneasy feeling that I don't know what the heck any given
decimal operation is going to do. It's probably justified in
this case, though.

-- 
Greg