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Author tim.peters mark.dickinson, rhettinger, serhiy.storchaka, skrah, steven.daprano, tim.peters 2018-03-20.01:49:36 -1.0 Yes <1521510578.75.0.467229070634.issue33089@psf.upfronthosting.co.za>
Content
```Mark, how about writing a clever single-rounding dot product that merely _detects_ when it encounters troublesome cases?  If so, it can fall back to a (presumably) much slower method.  For example, like this for the latter:

def srdp(xs, ys):
"Single rounding dot product."
import decimal
from decimal import Decimal, Inexact
# XXX check that len(xs) == len(ys)
with decimal.localcontext(decimal.ExtendedContext) as ctx:
ctx.traps[Inexact] = True
total = Decimal(0)
for x, y in zip(map(Decimal, xs), map(Decimal, ys)):
while True:
try:
total += x * y
break
except Inexact:
ctx.prec += 1
return float(total)

So it just converts everything to Decimal; relies on decimal following all the IEEE rules for special cases; retries the arithmetic boosting precision until decimal gets an exact result (which it eventually will since we're only doing + and *); and relies on float() to get everything about final rounding, overflow, and denorms right.  If that doesn't work, I'd say it's a bug in the decimal module ;-)

I'd bet a dollar that, in real life, falling back to this would almost never happen, outside of test cases.```
History
Date User Action Args
2018-03-20 01:49:38tim.peterssetrecipients: + tim.peters, rhettinger, mark.dickinson, steven.daprano, skrah, serhiy.storchaka