[C++-sig] What if a Python override returns several values?

[David Abrahams]

Alex Mohr <amohr at pixar.com> writes:

> Clearly that's a true statement, but I don't see where boost::tuple is 
> coming from.  My eyes must be broken.  I'll copy-paste the original 
> code, quoted earlier in this message.
>
> void g(int& a, int& b) {
>      boost::python::tuple tup = this->get_override("g")();
>      a = ...extract an int from tup[0]...
>      b = ...extract an int from tup[1]...
> }
>
> Compare this with the code you posted:
>
>  >>     python::tuple tup = this->get_override("g")();
>  >>     a = extract<t1>(tup[0]);
>
> The poster says the error message he gets is:
>
> TypeError: No registered converter was able to extract a C++ reference
> to type boost::python::tuple from this Python object of type tuple
>
> Where is boost::tuple coming into play?  

Nowhere; I think it's my eyes that are broken, sorry!

> I didn't see that at all 
> anywhere in the thread.  In my reply to the poster, I removed the 
> boost::python:: prefix from my code, so I mentioned "tuple" directly. 
> Perhaps that's what was causing confusion?  I meant boost::python::tuple.
>
> Regardless, unless I'm reading it wrong (please show me how!) I believe 
> that the code you posted is identical to the original posted code.

Okay.  Does it work if you don't use copy-initialization?

   boost::python::tuple tup(this->get_override("g")());

?

Does this work?

   boost::python::object t = this->get_override("g")();
   boost::python::tuple tup(t);

what about:

   boost::python::tuple tup 
     = this->get_override("g")().as<boost::python::tuple>();

??

>>>> There's almost never a reason to use call<> anymore!  I should
>>>> deprecate it.
>>> Please don't -- I use it in some low-level code.
>> 
>> Do you actually need it?
>
> I think so.  It's in some template code, where the return type is a 
> template parameter, and is allowed to be void, and IIRC, the problem was 
> that saying "return extract<void>(callable(...));" doesn't work. 
> Whereas saying "return call<void>(callable.ptr(), ...));" does.  I guess 
> I'd prefer to keep call<> around so I don't have to specialize the whole 
> thing for the case of void.
>
> I can try reworking it to avoid using call later in the week and get 
> back to you.

Don't bother; I understand your problem.  You need a facility
something more like get_override and method_result, which the library
doesn't supply for your case.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com