[C++-sig] Re: C++ Python Scope Problems

[David Abrahams]

"Ralf W. Grosse-Kunstleve" <rwgk at yahoo.com> writes:

> --- Chris Targett <xin at xlevus.net> wrote:
>
>> I see. But if i do:
>> 
>> 	import libHg 	# name of C++ library
>> 	foo = libHg.Test()
>> 	print id(foo.var1), foo.var1
>> 	foo.var1 += 1
>> 	print id(foo.var1), foo.var1
>> 
>> I get
>> 	135523772 0
>> 	135523760 1
>> 
>> Which clearley shows the varible is 'rebound'
>
> I got me worried..., but what you show above works correctly as one would
> expect. However, it has not much to do with your original problem. When you
> don't get the expected result you are doing something far more complex:
>
>         PyObject *pFunc = getPyFunc("testScript","testFunction");
>         call<void>(pFunc, this);
>
> David, please correct me if I am wrong: I think the call<>() must create a
> temporary Python object with a copy of "this". The temporary disappears at the
> end of the function call.

Correct.

> David, is there a way to create a temporary with a pointer to "this", e.g.
> call<void>(pFunc, *this);
> ?

Of course:

         call<void>(pFunc, boost::python::ptr(this));

> To the OP: as in real life, there are good and bad neighborhoods. In
> my judgement you are in a bad neighborhood. If you call Python from
> C++ I am wondering why you are in C++ in the first place. There will
> not be much of a speed gain compared to a pure Python solution.

Not everybody does this for speed, though.

> Here is a simple rule that works very well for me:
>
>   Never call Python from C++.

It doesn't work for anyone who has a C++ library with virtual
functions that they want to override from Python.  It is often
acceptable to have your Python subclasses of some base be slow, since
your C++ derived classes will be fast.  If speed matters you can start
moving those subclasses into C++.

Even if you're not writing subclasses, similar arguments apply to any
scenario where you call Python from C++ (there's an isomorphism in
there ;->).

> This side-steps a lot of potential surprises like your original
> problem.  In addition, I'd bet your code will be better organized.

It probably does help to keep things organized, and it's a good idea
when applicable, especially if performance is very important to you.
But it doesn't apply to every valid use case.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com