Vedran Čačić <ved...@gmail.com> added the comment:

> use more digits to manage rounding in decimal base, not only one but more (i 
> should think better and experiment on how many)

You don't have to. It's infinitely many. :-P Think, how many decimal digits 
would you need to accurately round numbers to a closest third (one trinary 
digit)? Here are some decimal digits: 2.166666666. If the next digit is 5, then 
it rounds to 2.0. If it is 7, it rounds to 2.1 (base 3). If it is 6, you still 
don't know anything. It can go arbitrarily far. Of course, the probability is 
lower with every digit, and at some point it becomes acceptable (you said for 
yourself it's acceptable even with one extra digit), but it's not 
mathematically correct.

And that one bit was just an illustration. In real life, 64-bit machines 
usually use at least 80-bit precision, so 16 extra bits. But it doesn't help 
your case, for the above reasons: this is simply not decimal rounding.

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<https://bugs.python.org/issue41598>
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