[Python-Dev] Decimal & amp; lt; -& amp; gt; float comparisons in py3k. (original) (raw)

Steven D'Aprano steve at pearwood.info
Sun Mar 21 23:32:20 CET 2010

On Mon, 22 Mar 2010 08:47:53 am Greg Ewing wrote:

Nick Coghlan wrote: > That's fine - binary floats are surprising. That's why Decimal > exists in the first place.

This argument could equally well be used the other way -- someone using Decimal is doing so precisely because they don't want to be surprised, in which case they would probably prefer to get an exception.

Then they're in for a terrible, terrible disappointment. Rounding issues don't go away because you're using Decimal instead of float, and I can't imagine anyone would like an exception in the following cases:

Decimal(1)/Decimal(3)*Decimal(3) == Decimal(1) False Decimal(2).sqrt()**Decimal(2) == Decimal(2) False Decimal(1028)+Decimal(1)-Decimal(1028) == Decimal(1) False

Rounding isn't the only surprise:

x = Decimal("NAN"); x == x False

Decimals are floats, but using radix 10 instead of radix 2.

-- Steven D'Aprano

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