Comparing float and decimal
Gerhard Häring
gh at ghaering.de
Tue Sep 23 14:45:33 CEST 2008
D'Arcy J.M. Cain wrote:
> I'm not sure I follow this logic. Can someone explain why float and
> integer can be compared with each other and decimal can be compared to
> integer but decimal can't be compared to float?
>
>>>> from decimal import Decimal
>>>> i = 10
>>>> f = 10.0
>>>> d = Decimal("10.00")
>>>> i == f
> True
>>>> i == d
> True
>>>> f == d
> False
I can give you the technical answer after reading the sources of the
decimal module: you can only compare to Decimal what can be converted to
Decimal. And that is int, long and another Decimal.
Everything else will return False when comparing.
> This seems to break the rule that if A is equal to B and B is equal to
> C then A is equal to C.
Yes, but only if comparison from type(A) to type(C) is supported at all.
Instead of raising ValueError or NotImplementedError, the decimal
module returns False here.
-- Gerhard
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