if instance exists problem ..

[Gabriel Genellina]

En Thu, 18 Oct 2007 00:07:34 -0300, Steven D'Aprano  
<steve at REMOVE-THIS-cybersource.com.au> escribió:

> On Thu, 18 Oct 2007 03:00:56 +0000, Steven D'Aprano wrote:
>
>> On Wed, 17 Oct 2007 23:12:15 +0000, Paul Hankin wrote:
>>
>>> 'if x' doesn't test if x exists, it tests if x when cast to a bool is
>>> True.
>>
>> To be pedantic:
>>
>> Python doesn't have type casts. bool(x) doesn't cast x as a bool, it
>> creates a brand new Boolean object from x.
>
> Actually, to be even more pedantic, True and False are both singletons,
> and so bool() doesn't actually create a new Boolean object but reuses the
> existing ones.

To be even more pedantic, 'if x' does not invoke the builtin bool(),  
instead it uses PyObject_IsTrue, which returns a plain C integer, not a  
Python bool object.

-- 
Gabriel Genellina