[Python-Dev] Decimal <-> float comparisons in py3k.

Greg Ewing greg.ewing at canterbury.ac.nz
Thu Mar 18 00:58:40 CET 2010

Guido van Rossum wrote:
> in some
> "intuitive complexity" sense an int is a simpler type than a float and
> a float is a simpler type than a Decimal

I don't think this analogy holds. In a mathematical sense,
ints are a subset of reals, but binary and decimal floats
are just alternative approximate representations of reals,
neither one being inherently preferable over the other.

One could argue that since all binary floats are exactly
representable in decimal but not vice versa, decimal should
be regarded as the wider type. But even this doesn't hold
when you have a limited number of decimal digits available,
which you always do at any given moment with the Decimal

And even if there are enough digits, an exact conversion
mightn't be what you really want. This problem doesn't arise
with int->float conversion -- there is only one obvious way
of chopping it to fit.


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