[C++-sig] Re: compile errors with embedding example from tutorial

[Faheem Mitha]

On Sun, 27 Jun 2004 23:46:06 -0500, Chad Austin <caustin at gmail.com> wrote:

> Hi Faheem,

Hi Chad,

Thanks for replying.

> I ran into that too.  In short, the line with "handle<>( PyRun_String"
> isn't being understood how you want it too...  You can surround that
> expression with parenthesis, and it should work:
>
> (handle<>( PyRun_String("hello = file('hello.txt', 'w')\n"
>                         "hello.write('Hello world!')\n"
>                           "hello.close()", Py_file_input,
>                           main_namespace.get(), main_namespace.get()) ));

Thanks. Indeed it does. I hope the tutorial can be changed
appropriately, unless it already has been, of course. Any idea why
this is necessary? It isn't in the other two places <handle> is used.

I can load my module, but when I try to run the function, I get

NameError: name 'file' is not defined

Any idea about this?

If you know of other places the tutorial needs to be changed, I would
appreciate being informed of them.

I think it might be useful if I said explicitly what it is I am trying
to do here, and ask for suggestions.

I'm writing an extension module, and want to call a python function,
namely array from the numarray.strings module. This is part of the
Python Numarray library. The question is how to do this.

I have tried doing this with callbacks and it seems to work fine. The
trouble with callbacks is that it means I have to have a python
wrapper with an argument slot for every function I want to
import. This is clunky.

Suppose I have three functions, say f1, f2, f3.

Then I have f1 calling f2 calling f3. 

f1 -> f2 -> f3

Suppose f3 calls some python function (say `foo') from an external
module or something.

Then if I rewrite f3 using Boost.Python, I still need a wrapper, say
f1', to pass `foo' as an argument. Then I need

f1 -> f2 ->  f3' -> f3

I suppose I don't need `foo' passed down from f1 because if I import
foo into global namespace into the same module where I define f1, f2,
f3', then Python's scoping rules will ensure that if I use `foo' as an
argument to f3' then it will be picked up from the global namespace.

Furthermore, if I decide to rewrite f2 into Boost.Python, then I would
have to create another Python wrapper f2' for f2, and pass f3' as an
argument to f2' (ugh). So I'd get the following

f1 -> f2' -> f2 -> f3' -> f3 

I'd rather have 

f1 -> f2 -> f3

where f2 and f3 only exist as C++ functions. Much cleaner.

So I thought that perhaps I could imbed the interpreter inside (say)
f3, and persuade it to make `foo' available to me inside C++. I don't
know if this is even possible. I have seen no examples of this. In
fact, I have seen no examples of using the python interpeter inside
extension modules. In fact, I've been rather unsuccessful so far in
getting anything related to embedding to work.

Am I missing something? Thanks in advance for any information.

                                                            Faheem.