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

Antoine Pitrou solipsis at pitrou.net
Thu Mar 18 19:44:52 CET 2010

Glenn Linderman <v+python g.nevcal.com> writes:

For all the reasons that mixed decimal and float arithmetic is bad, mixed decimal and float comparisons are also bad. To do proper comparisons, you need to know the number of significant digits of both numbers, and the precision and numeric ranges being dealt with by the application.

What is a "proper comparison"? A comparison is a lossy operation by construction (it returns only 1 bit of output, or 2 bits for rich comparison), so the loss in precision that results from mixing float and decimal operands should not be a concern here.

Unless there are situations where the comparison algorithm might return wrong results (are there?), I don't see why we should forbid such comparisons.

Seems like it would be better to raise an exception, and in the documentation for the exception point out that turning off the exception (if it should be decided that that should be possible, which could be good for compatibility), would regress to the current behavior, which doesn't sort numerically, but by type.

Regressing to an useless behaviour sounds like a deliberate annoyance. I don't think this proposal should be implemented.

Regards

Antoine.

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