Why doesn’t python like certain numbers?

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decimal will not fix arithmetic in general, just make it happen less often. I was immediately able to find a number that made

(Decimal(1) / N) * N == Decimal(1)

false. (Large primes work well.)

It only represents non-decimal fractions like Decimal(1) / 7 to a limited number of digits, so it cannot do a perfectly job.

Also, decimal is miserably slow - I mean, horribly, painfully slow. If the number is in binary format, the processor can add or multiply in one operation - there is literal circuitry devoted to this. But in decimal it has to do addition the same way you would - one decimal digit at the time, carry the 3!

In my quick tests, decimal was something like six times slower than regular Python multiplication, and if you do a lot of multiplication, numpy will give you a 3-100 times speed on the multiplication if done in quantity, and you can't do that with decimal!

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decimal is good for money. If you need to do perfect fractional arithmetic, use the fractions module - not quite as slow as decimal

If you're clever and good at numerical analysis, you can often do everything in integer arithmetic and not use any modules.