cxx-users — Discuss the use and improvement of PyCXX

Re: How to program a callback with PyCXX?

[Barry S. <ba...@ba...>]

On May 18, 2008, at 16:57, Sven Alisch wrote:

> Hi members!
>
> I am new to this list, so sorry for my bad english. I hope you  
> understand me.
>
> First of all: Thanks for your great and well documented project. It  
> is very
> simple to start programming a wrapper. I tried a little bit and it  
> works.
>
> Now I want to do following. I want to program a callback function.  
> This means
> I want to give my C++-Methods a Python-Function (like a c function  
> pointer).
> Then I want to call this Python-Function like a callback-function  
> in C. But
> how to do this? Could anybody be so sind to give me a hint?

Sorry for the long time in replying...

Do you want to call python code from C++?

Create a Py::Callable object and use its apply() method to call
the python code.

The pysvn project uses pycxx and uses a lot of its features.
Its project page is http://pysvn.tigris.org. It may have examples
that an useful to you.

Barry

How to program a callback with PyCXX?

[Sven A. <sv...@gm...>]

Hi members! 

I am new to this list, so sorry for my bad english. I hope you understand me. 

First of all: Thanks for your great and well documented project. It is very 
simple to start programming a wrapper. I tried a little bit and it works.

Now I want to do following. I want to program a callback function. This means 
I want to give my C++-Methods a Python-Function (like a c function pointer). 
Then I want to call this Python-Function like a callback-function in C. But 
how to do this? Could anybody be so sind to give me a hint?

Thanks a lot,

Sincerly,
Sven

Re: Exception handling

[Barry S. <ba...@ba...>]

On Oct 18, 2007, at 04:40, Tim Docker wrote:

> I've just been taking a look at pyCxx, and it's looks quite nice.
>
> I notice that in Objects.hxx, there's lot's of code snippets that  
> look like:
>
>             if (PyErr_Occurred()) throw Exception();
>
> Would it be better to call a function that interrogates the  
> exeception status, and throws an exception of the appropriate  
> derived type? That way you could catch more specifically in the C++  
> code.
>

Having re-read the code that would be better. Would you like to  
submit a patch to
do this?

>
> Also, is there a reason why Exception doesn't derive from  
> std::exception?

No reason, the code is older then std::exception and I guess it  
should be updated
so long as there is no problem introduced by doing this.

Barry

Exception handling

[Tim D. <ti...@ma...>]

I've just been taking a look at pyCxx, and it's looks quite nice.
=20
I notice that in Objects.hxx, there's lot's of code snippets that look
like:
=20
            if (PyErr_Occurred()) throw Exception();
=20
Would it be better to call a function that interrogates the exeception
status, and throws an exception of the appropriate derived type? That
way you could catch more specifically in the C++ code.
=20
Also, is there a reason why Exception doesn't derive from
std::exception?
=20
thanks,
=20
Tim

Re: PyCXX problem (update)

[Barry S. <ba...@ba...>]

On Mar 15, 2007, at 13:41, Gregor Mirai wrote:

> Hello,

There is a PyCXX mailing list that we can use for this. I've CC'ed to  
get
this useful info onto the list.

>
> I have traced the problem down to the GCC compiler.
>
> For instance using more than one shared object file from python  
> works ok when
> they are compiled on Solaris machine.
>
> The cause of the problem (when compiling with GCC) are multiple  
> exception
> typeinfos that have different addresses which does not match well  
> when python
> tries to catch exceptions. In effect python doesn't catch them and the
> program terminates.
>
> It seems this is a well known bug, that will be fixed in upcoming  
> version 4.2.
> of GCC compiler.
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19664

That's nasty.

>
> I am not sure if there is any workaround for this issue. The  
> intermediate
> solution would be to catch all exceptions in python extension  
> libraries and
> return some sort of exception info to the caller.

THe code does catch all exceptions and return to python. I guess that  
the compiler
bug is preventing C++ exceptions from working. And that means that  
the translation
from C++ to Python exceptions is broken.

Barry

Re: Embedding Python

[Barry S. <ba...@ba...>]

On Dec 8, 2006, at 17:07, carlos choy wrote:

> I looked at the Demo folder and found usage of the Callable  
> interface in
> rangetext.cxx.
> In version 5.36, rangetest.cxx has the following lines (starting at  
> line
> 29):
>
> *******************************************
>     RangeSequence r2(1, 10, 2);
>     if(r2[1] != Py::Int(3))
>         return "RangeSequence check failed. ";
>
>     debug_check_ref_queue();
>
>     // calling an extension object method using getattr
>     Py::Callable w(r2.getAttr("amethod"));
>     Py::Tuple args(1);
>     Py::Int j(3);
>     args[0]=j;
>     Py::List answer(w.apply(args));
>     if(answer[0] != r2)
>         return ("Extension object test failed (1)");
>
>     if(answer[1] != args[0])
>         return ("Extension object test failed (2)");
> *******************************************
>
> I see that RangeSequence is a class extending a python sequence.   
> However, I
> don't see how "amethod" could be an attribute of it.  "amethod" is  
> a method
> of the range class, but this class seems to have nothing to do with
> RangeSequence.  How is this method an attribute of the extended class?


RangeSequence is Py::Object that points to an instance of range. See  
line 112 of range.hxx.


> Next, I see that 'w' is a Callable object, but I do not see how it  
> can have
> an 'apply' method.  Is the 'apply' method putting the arguments in  
> 'args'
> into the method "amethod"?

It works the same way that apply builtin works in python. Call the  
function that w contains
passing it args and store the returned value in answer.

>
> Lastly, I don't see at all how the Callable object is calling back  
> into a
> python script.

The apply() functions calls into python.

>
> Can you please give me some explanations?  Sorry if this all seems  
> rather
> elementary, but I am trying my best to understand it.


No problem.


Barry

RE: Embedding Python

[carlos c. <hap...@ho...>]

Thanks for the prompt reply.

I looked at the Demo folder and found usage of the Callable interface in
rangetext.cxx.
In version 5.36, rangetest.cxx has the following lines (starting at line
29):

*******************************************
     RangeSequence r2(1, 10, 2);
     if(r2[1] != Py::Int(3))
         return "RangeSequence check failed. ";

     debug_check_ref_queue();

     // calling an extension object method using getattr
     Py::Callable w(r2.getAttr("amethod"));
     Py::Tuple args(1);
     Py::Int j(3);
     args[0]=j;
     Py::List answer(w.apply(args));
     if(answer[0] != r2)
         return ("Extension object test failed (1)");

     if(answer[1] != args[0])
         return ("Extension object test failed (2)");
*******************************************

I see that RangeSequence is a class extending a python sequence.  However, I
don't see how "amethod" could be an attribute of it.  "amethod" is a method
of the range class, but this class seems to have nothing to do with
RangeSequence.  How is this method an attribute of the extended class?

Next, I see that 'w' is a Callable object, but I do not see how it can have
an 'apply' method.  Is the 'apply' method putting the arguments in 'args'
into the method "amethod"?

Lastly, I don't see at all how the Callable object is calling back into a
python script.

Can you please give me some explanations?  Sorry if this all seems rather
elementary, but I am trying my best to understand it.

 >From: Barry Scott <ba...@ba...>
 >Reply-To: Discuss PyCXX use and improvement
 ><cxx...@li...>
 >To: Discuss PyCXX use and improvement <cxx...@li...>
 >Subject: Re: Embedding Python
 >Date: Sun, 3 Dec 2006 19:33:04 +0000
 >
 >
 >On Dec 1, 2006, at 18:54, carlos choy wrote:
 >
  I am thinking of using PyCXX principally for embedding Python.
 >
 >This works. You will need to create a module using PyCXX if you
 >wish the python code to be able to call you back.
 >
 >If you use threads there is extra work in the init and in the module
 >you will need to worry about.
 >
 >
  Here is some
  basic code for embedding that I intend to use:

  ****************************************************************
  #include <Python.h>

  int main(int argc, char *argv[])
  {
      PyObject *pName, *pModule, *pDict, *pFunc, *pValue;

 >
 >Use appropriate Py:: objects.
 >
      if (argc < 3)
      {
          printf("Usage: exe_name python_source function_name\n");
          return 1;
      }

 >
 >	Give python an argc and argv before calling init.
 >
      // Initialize the Python Interpreter
      Py_Initialize();

      // Build the name object
      pName = PyString_FromString(argv[1]);
 >
 >	Py::String name( argv[1] );
 >

      // Load the module object
      pModule = PyImport_Import(pName);
 >
 >	Py::Module module( PyImport_Import( name ) );
 >
      // pDict is a borrowed reference
      pDict = PyModule_GetDict(pModule);
 >
 >Py::Dict dict( module.getDict() );
 >

      // pFunc is also a borrowed reference
      pFunc = PyDict_GetItemString(pDict, argv[2]);
 >
 >	Py::Callable func( dict[ Py::String( argv[2] ) ] );
 >

      if (PyCallable_Check(pFunc))
      {
          PyObject_CallObject(pFunc, NULL);
      } else
      {
          PyErr_Print();
      }
 >
 >No need to do the if an exception will be raise if an objects type
 >does not match.
 >
 >	// no args
 >	Py::Tuple args( 0 );
 >
 >	func.apply( args );
 >
      // Clean up
      Py_DECREF(pModule);
      Py_DECREF(pName);
 >
 >	No need to call dec.
 >	But you will need to make sure that the Py:: onjects go out of scope
 >so that
 >	they are deleted before calling finalize.
 >

      // Finish the Python Interpreter
      Py_Finalize();

      return 0;
  }
  **********************

  I assume the above will work.  Is there a more PyCXX way of doing
  this?
 >
 >See comments in line above.
 >

  Lastly, I need to have callbacks into C++ code from Python.  The
  Callable
  interface is documented, but samples.  Can someone please post
  examples of
  how to use it?

  _________________________________________________________________
 >
 >Define a module and add functions and classes. Look at the example
 >that comes with PyCXX in the Demo folder. For example look at
 >range.hxx and .cxx.
 >
 >If you want a full scale example look at the source of the Python SVN
 >extension, pysvn
 >at http://pysvn.tigris.org - download a source kit add you will see
 >examples of calling C++ from
 >python and calling python from C++.
 >
 >Barry
 >
 >
 >
 >-------------------------------------------------------------------------
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 >https://lists.sourceforge.net/lists/listinfo/cxx-users

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Re: Embedding Python

[Barry S. <ba...@ba...>]

On Dec 1, 2006, at 18:54, carlos choy wrote:

> I am thinking of using PyCXX principally for embedding Python.

This works. You will need to create a module using PyCXX if you
wish the python code to be able to call you back.

If you use threads there is extra work in the init and in the module
you will need to worry about.


> Here is some
> basic code for embedding that I intend to use:
>
> ****************************************************************
> #include <Python.h>
>
> int main(int argc, char *argv[])
> {
>     PyObject *pName, *pModule, *pDict, *pFunc, *pValue;
>

Use appropriate Py:: objects.

>     if (argc < 3)
>     {
>         printf("Usage: exe_name python_source function_name\n");
>         return 1;
>     }
>

	Give python an argc and argv before calling init.

>     // Initialize the Python Interpreter
>     Py_Initialize();
>
>     // Build the name object
>     pName = PyString_FromString(argv[1]);

	Py::String name( argv[1] );

>
>     // Load the module object
>     pModule = PyImport_Import(pName);

	Py::Module module( PyImport_Import( name ) );

>     // pDict is a borrowed reference
>     pDict = PyModule_GetDict(pModule);

Py::Dict dict( module.getDict() );

>
>     // pFunc is also a borrowed reference
>     pFunc = PyDict_GetItemString(pDict, argv[2]);

	Py::Callable func( dict[ Py::String( argv[2] ) ] );

>
>     if (PyCallable_Check(pFunc))
>     {
>         PyObject_CallObject(pFunc, NULL);
>     } else
>     {
>         PyErr_Print();
>     }

No need to do the if an exception will be raise if an objects type  
does not match.

	// no args
	Py::Tuple args( 0 );

	func.apply( args );

>     // Clean up
>     Py_DECREF(pModule);
>     Py_DECREF(pName);

	No need to call dec.
	But you will need to make sure that the Py:: onjects go out of scope  
so that
	they are deleted before calling finalize.

>
>     // Finish the Python Interpreter
>     Py_Finalize();
>
>     return 0;
> }
> **********************
>
> I assume the above will work.  Is there a more PyCXX way of doing  
> this?

See comments in line above.

>
> Lastly, I need to have callbacks into C++ code from Python.  The  
> Callable
> interface is documented, but samples.  Can someone please post  
> examples of
> how to use it?
>
> _________________________________________________________________

Define a module and add functions and classes. Look at the example
that comes with PyCXX in the Demo folder. For example look at  
range.hxx and .cxx.

If you want a full scale example look at the source of the Python SVN  
extension, pysvn
at http://pysvn.tigris.org - download a source kit add you will see  
examples of calling C++ from
python and calling python from C++.

Barry

Embedding Python

[carlos c. <hap...@ho...>]

I am thinking of using PyCXX principally for embedding Python.  Here is some 
basic code for embedding that I intend to use:

****************************************************************
#include <Python.h>

int main(int argc, char *argv[])
{
    PyObject *pName, *pModule, *pDict, *pFunc, *pValue;

    if (argc < 3)
    {
        printf("Usage: exe_name python_source function_name\n");
        return 1;
    }

    // Initialize the Python Interpreter
    Py_Initialize();

    // Build the name object
    pName = PyString_FromString(argv[1]);

    // Load the module object
    pModule = PyImport_Import(pName);

    // pDict is a borrowed reference
    pDict = PyModule_GetDict(pModule);

    // pFunc is also a borrowed reference
    pFunc = PyDict_GetItemString(pDict, argv[2]);

    if (PyCallable_Check(pFunc))
    {
        PyObject_CallObject(pFunc, NULL);
    } else
    {
        PyErr_Print();
    }

    // Clean up
    Py_DECREF(pModule);
    Py_DECREF(pName);

    // Finish the Python Interpreter
    Py_Finalize();

    return 0;
}
**********************

I assume the above will work.  Is there a more PyCXX way of doing this?

Lastly, I need to have callbacks into C++ code from Python.  The Callable 
interface is documented, but samples.  Can someone please post examples of 
how to use it?

_________________________________________________________________
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Welcome

[Barry S. <ba...@ba...>]

This list is for discussing the use and improvement of PyCXX - the  
Python C++ connection.

Barry