Just a couple of quick side comments on this; I haven't got my head around the whole mixed-operations idea yet.
On Fri, Mar 19, 2010 at 9:50 PM, Guido van Rossum <gu...@python.org> wrote: > There is one choice which I'm not sure about. Should a mixed > float/Decimal operation return a float or a Decimal? I'll just say that it's much easier to return a Decimal if you want to be able to make guarantees about rounding behaviour, basically because floats can be converted losslessly to Decimals. I also like the fact that the decimal module offers more control (rounding mode, precision, flags, wider exponent range) than float. In general, the correct semantics for an arithmetic operation are to produce a result that's equivalent to what would have been obtained by performing the operation to infinite precision and then doing a single round to fit that result into the output type format. This is trivially easy to do if mixed float and Decimal operations produce Decimal, and much much harder if they produce floats; if mixed-type operations produced floats we'd probably have to go with algorithms that involve two rounds (i.e., first coerce the Decimal to a float, then do the operation as usual on the two floats), and there would likely be some (small) numeric surprises as a result. > The implementation of __hash__ will be complicated, and it may make > sense to tweak the hash function of float, Fraction and Decimal to > make it easier to ensure that for values that can be represented in > either type the hash matches the equality. But this sounds a > worthwhile price to pay for proper embedding in the numeric tower. I don't think this is going to be a problem. I've implemented most of the scheme I outlined earlier (it's working for ints, floats and Decimals; still need to implement it for Fractions and complex numbers) and it seems to work just fine, with essentially no more code than was there before. I'll post a proof-of-concept patch when I've filled in the missing bits. Mark _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
