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




More information about the Python-list mailing list