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

[David Abrahams]

Alex Mohr <amohr at pixar.com> writes:

>> But if I have another virtual member function "void g(int& a, int &b)" 
>> which returns two integers, then I'd like the Python version to take no 
>> arguments and return a tuple (a,b). But how does the wrapper look like 
>> in this case? I would have expected something like:
>> 
>>      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]...
>>      }
>> 
>> But when I run this code I get the following error at runtime:
>> 
>> TypeError: No registered converter was able to extract a C++ reference 
>> to type boost::python::tuple from this Python object of type tuple
>
> Not sure exactly what's going on here, but you might be able to find out 
> by looking at override.hpp.  Look at method_result -- that's what you're 
> getting back from get_override.
>
> Off the top of my head, I think this would probably work for you:
>
> tuple tup = call<tuple>(this->get_override("g").ptr());

> a = ...extract an int from tup[0]
> b = ...extract an int from tup[1]


I don't know why people always use ugly low-level interfaces when they
can use beautiful high-level ones:

    python::tuple tup = this->get_override("g")();
    a = extract<t1>(tup[0]);
    .
    .
    .

There's almost never a reason to use call<> anymore!  I should
deprecate it.

I'm not trying to be critical, but this really concerns me.  I see
this sort of thing all the time.  If you could help me understand
why you were moved to suggest doing it the hard way, I would
appreciate it.

> It's certainly not as nice as what you wrote, 

But it could be. :)

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