On Fri, 9 Oct 2020 at 23:41, Tim Peters <tim.pet...@gmail.com> wrote: > But the decimal spec takes a different approach, which Python's docs > don't explain at all: the otherwise-mysterious ROUND_05UP rounding > mode. Quoting from the spec: > > http://speleotrove.com/decimal/damodel.html > ... > The rounding mode round-05up permits arithmetic at shorter > lengths to be emulated in a fixed-precision environment without > double rounding. For example, a multiplication at a precision of 9 > can be effected by carrying out the multiplication at (say) 16 > digits using round-05up and then rounding to the required length > using the desired rounding algorithm. > > In your original example, 1.01 * 1.46 rounds to 4-digit 1.474 under > ROUND_05UP. and then `quantize()` can be used to round that back to 1, > 2, or 3 digits under any rounding mode you like. > > Or, with your last example, > > >>> with decimal.localcontext() as ctx: > ... ctx.rounding = decimal.ROUND_05UP > ... r = D('1.00000000000001')*D('1.49999999999996') > >>> r > Decimal('1.499999999999974999999999999') > >>> r.quantize(D('.00000000000001')) > Decimal('1.49999999999997')
And can't be this the default of decimal? For what I know, this is the default of BigDecimal in Java: > public BigDecimal multiply(BigDecimal multiplicand) > > Returns a BigDecimal whose value is (this × multiplicand), and whose scale is > (this.scale() + multiplicand.scale()). > Parameters:multiplicand - value to be multiplied by this > BigDecimal.Returns:this * multiplicand > > > public BigDecimal multiply(BigDecimal multiplicand, > MathContext mc) > > Returns a BigDecimal whose value is (this × multiplicand), with rounding > according to the context settings. Example online: http://tpcg.io/5axMxUQb _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/VPAV3T2XN4DARL5RNBNCYFP5IP4BGYAX/ Code of Conduct: http://python.org/psf/codeofconduct/
