From: Robert Kern on
On 2010-03-05 17:48 PM, Jack Diederich wrote:
> On Fri, Mar 5, 2010 at 6:09 PM, Steven D'Aprano
> <steve(a)> wrote:
>> On Fri, 05 Mar 2010 15:58:01 -0500, Jack Diederich wrote:
>>>>> So, the pythonic way to check for True/False should be:
>>>>>>>> 1 is True
>>>>> False
>>>> Why do you need to check for True/False?
>>> You should never check for "is" False/True but always check for
>>> equality. The reason is that many types support the equality (__eq__)
>>> and boolen (__bool__ in 3x) protocols. If you check equality these will
>>> be invoked, if you check identity ("is") they won't.
>> Never say never.
>> If you specifically want to test for True or False themselves, accepting
>> no substitutes, then using "is" is the obvious way, and using "==" is
>> clearly and obviously wrong because it does accept substitutes:
>>>>> 1.0 == True
>> True
>>>>> decimal.Decimal(0, 1) == False
>> True
> Yes, obviously if you _really_ mean to test if something has the
> object identity of True or False then an "is" test is the way to go.
> I'm just not sure why you would ever do that. Also, I'm not sure how
> your assertion matches up with the examples; The examples test for
> equality with a float that returns true for __eq__ and a Decimal that
> returns false for __eq__.

No, both comparisons return True. Decimal(0,1) is equal in value to 0 (and thus
False). Comparing it to False using __eq__ returns True.

> Both "1.0" and "Decimal(0, 1)" will return
> False if the test is "is True" or "is False."

Yes. That is exactly what he is asserting.

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

From: Bruno Desthuilliers on
Rolando Espinoza La Fuente a écrit :
> On Fri, Mar 5, 2010 at 2:32 PM, mk <mrkafk(a)> wrote:
>> Arnaud Delobelle wrote:
>>>>>> 1 == True
>>> True
>>>>>> 0 == False
>>> True
>>> So what's your question?
>> Well nothing I'm just kind of bewildered: I'd expect smth like that in Perl,
>> but not in Python.. Although I can understand the rationale after skimming
>> PEP 285, I still don't like it very much.
> So, the pythonic way to check for True/False should be:
>>>> 1 is True
> False
>>>> 0 is False
> False
> instead of ==, right?

Nope. The pythonic way is to check for truth value - not for True or
False -, and to only use the identity test when wanting to test for

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