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

[Chris Angelico]

On Sat, Mar 8, 2014 at 2:55 PM, Paul Du Bois <paul.dubois at gmail.com> wrote:
> On Fri, Mar 7, 2014 at 6:15 PM, Guido van Rossum <guido at python.org> wrote:
>>
>> Or, Decimal(repr(some_float)), which DWIM.
>
>
> Because I haven't seen anyone else bring this up, using a non-exact
> conversion breaks the invariant "Decimal(f)==f".
>
> There are so many pairs of numeric types that break that invariant that it
> might not be a big deal; but in all other cases the invariant is broken
> because it is theoretically impossible.
>
> from decimal import Decimal
> f = 2.2
> print(Decimal(f)==f)
> print(Decimal(repr(f))==f)

How is Decimal==float implemented? Is it by calling Decimal(rhs) and
then comparing? If so, changing how Decimal(float) works won't break
the invariant, as it'll make the same conversion each time.

ChrisA