[C++-sig] Checking for or null PyObject* or PyNone

[Jim Bosch]

On Sun, 2010-02-07 at 11:47 +0100, Murray Cumming wrote:
> Doing this
>   boost::python::object obj = get_some_object();
>   if(obj)
>   {
>      //do something
>   }
> seems to check a bool value inside the underlying PyObject, though I
> guess that could throw an exception if it doesn't contain actually
> contain a bool. Is that right?
> The reference documentation is not very clear about this:
> http://www.boost.org/doc/libs/1_42_0/libs/python/doc/v2/object.html
> 
> 
> I had wrongly guessed that it would check if the underlying PyObject*
> was 0. I did this because I had wrapped a PyObject* in a
> boost::python::object for convenience, without checking for 0 first.
> 

I believe the checking happens typically in boost::python::handle, which
is an intermediate between PyObject* and boost::python::object.  When
you construct a handle, you can pass wrap it with null_ok if you don't
want it to throw on NULL; otherwise I think it would.  How are you
making the boost::python::object?  I always construct a handle first.

> Is there any correct way to check, other than doing
>   if(obj.ptr())
> ?
> 
> 
> And what's the correct way to check for PyNone other than 
>   if (obj == boost::python::object())
> ?
> 

I think that's the standard way.  I suppose

if (obj.ptr() == Py_None)

might be faster on some compilers, but it's almost certainly not worth
worrying about.


Jim Bosch