[Python-Dev] Decimal <-> float comparisons in py3k. (original) (raw)

Mark Dickinson dickinsm at gmail.com
Sat Mar 20 21:38:46 CET 2010

On Sat, Mar 20, 2010 at 7:56 PM, Guido van Rossum <guido at python.org> wrote:

I propose to reduce all hashes to the hash of a normalized fraction, which we can define as a combination of the hashes for the numerator and the denominator. Then all we have to do is figure fairly efficient ways to convert floats and decimals to normalized fractions (not necessarily Fractions). I may be naive but this seems doable: for a float, the denominator is always a power of 2 and removing factors of 2 from the denominator is easy (just right-shift until the last bit is zero). For Decimal, the unnormalized denominator is always a power of 10, and the normalization is a bit messier, but doesn't seem excessively so. The resulting numerator and denominator may be large numbers, but for typical use of Decimal and float they will rarely be excessively large, and I'm not too worried about slowing things down when they are (everything slows down when you're using really large integers anyway).

I am worried about slowing things down for large Decimals: if you can't put Decimal('1e1234567') into a dict or set without waiting for an hour for the hash computation to complete (because it's busy computing 10**1234567), I consider that a problem.

But it's solvable! I've just put a patch on the bug tracker:

http://bugs.python.org/issue8188

It demonstrates how hashes can be implemented efficiently and compatibly for all numeric types, even large Decimals like the above. It needs a little tidying up, but it works.

Mark

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