Comparing float and decimal

[Robert Lehmann]

On Tue, 23 Sep 2008 07:20:12 -0400, 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?

In comparisons, `Decimal` tries to convert the other type to a `Decimal`. 
If this fails -- and it does for floats -- the equality comparison 
renders to False. For ordering comparisons, eg. ``D("10") < 10.0``, it 
fails more verbosely::

  TypeError: unorderable types: Decimal() < float()

The `decimal tutorial`_ states:

  "To create a Decimal from a float, first convert it to a string. This  
  serves as an explicit reminder of the details of the conversion 
  (including representation error)."

See the `decimal FAQ`_ for a way to convert floats to Decimals.


>>>> from decimal import Decimal
>>>> i = 10
>>>> f = 10.0
>>>> d = Decimal("10.00")
>>>> i == f
> True
>>>> i == d
> True
>>>> f == d
> False
> 
> 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.

I don't see why transitivity should apply to Python objects in general.

HTH,

.. _decimal tutorial: http://docs.python.org/lib/decimal-tutorial.html
.. _decimal FAQ: http://docs.python.org/lib/decimal-faq.html

-- 
Robert "Stargaming" Lehmann