Re: [Swig-user] Python C++ extension (multi threaded environment) (Somethign useful ? )

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I used SWIG_NewPointerObj method to create a PyObject. It removed all my
exceptions and seems to be working fine. 

[code]
PyObject *argobj = SWIG_NewPointerObj(SWIG_as_voidptr(&msg2),
SWIGTYPE_p_RFAMessageWrapper, SWIG_POINTER_NEW |  0 );
 try {
  arglist = Py_BuildValue("(O)",argobj);             // Build argument
list
  PyEval_CallObject(func,arglist);     // Call Python
 } catch (...) {
  cout<<"Unknown exception..."<<endl;
 }
[\code]

Callback is called properly. 

But now, in Python code

Access to the passed object raises exception. I am expecting
RFAMessageWrapper object in python code. But using SWIG_NewPointerObj
passes SwigPyObject to python instead.

Python call back function is 
[code]
def callback_fun(msg):
    try:
        print "RFAMessage received for service:"+msg.service
    except Exception:
        print "Exception handled"
        print type(msg)
        traceback.print_exc(file=sys.stdout)
[/code]

Result:

[code]
Exception handled
<type 'SwigPyObject'>
Traceback (most recent call last):
  File "<stdin>", line 3, in callback_fun
AttributeError: 'SwigPyObject' object has no attribute 'service'
[/code]

I am expecting the object type as RFAMessageWrapper here.  Even when the
PyObject created in C code uses SWIGTYPE_p_RFAMessageWrapper as the
typeinfo, still the object created is not RFAMessageWrapper.

What should I pass to callback  PyEval_CallObject(func,arglist);     so
that I can access RFAMessageWrapper object in Python code. 

RFAMessageWrapper class is wrapped using SWIG properly.

Any help is appreciated.

Regards, Alok

________________________________

From: Jadhav, Alok 
Sent: Tuesday, December 08, 2009 9:42 AM
To: Jadhav, Alok; 'Brian Cole'
Cc: 'swi...@li...'
Subject: RE: [Swig-user] Python C++ extension (multi threaded
environment) (Somethign useful ? )

Someone from Python-C++ mailing list told me that the way I create
argument list is incorrect. They did not have knowledge of SWIG so they
couldn't help more.

In the function below

RFAMessageWrapper msg2(msg);
 try {
  arglist = Py_BuildValue("(O)",msg2);             // Build argument
list
  //arglist = Py_BuildValue("(i)", blah);
  PyEval_CallObject(func,arglist);     // Call Python
 } catch (...) {
  cout<<"Unknown exception..."<<endl;
 }

The code   arglist = Py_BuildValue("(O)",msg2);    

Here I create msg2 object as 
RFAMessageWrapper msg2(msg); 

which is on stack. 

Other person told me that this is incorrect. I should create this object
as PyObject and then call Py_BuildValue function. It should be created
using PyObject_New. 
Since RFAMessageWrapper is already wrapped using SWIG, I believe there
should already be a function which would help me in creating a new
PyObject for RFAMessageWrapper. 

Is this correct?

Can someone tell me which function I should use to create a new PyObject
type object for RFAMessageWrapper?

Regards,
Alok

________________________________

From: Jadhav, Alok 
Sent: Tuesday, December 08, 2009 8:21 AM
To: Jadhav, Alok; Brian Cole
Cc: swi...@li...
Subject: RE: [Swig-user] Python C++ extension (multi threaded
environment)

I still don't have any progress. Funny thing is that, I am not even able
to catch this exception !!!!

I put a try catch block in C code like below 

RFAMessageWrapper msg2(msg);
 try {
  arglist = Py_BuildValue("(O)",msg2);             // Build argument
list
  //arglist = Py_BuildValue("(i)", blah);
  PyEval_CallObject(func,arglist);     // Call Python
 } catch (...) {
  cout<<"Unknown exception..."<<endl;
 }

While debugging, PyEval_CallObject(func,arglist);    is executed and it
raises exception but it is not cought in this code. 

I put a try catch block in my python callback function as well ... 

def callback_fun(msg):
    try:
        print "RFAMessage received for service:"+msg.service
    except Exception:
        print "Exception handled"

Here again exception is not handled. 

msg.service accesss raises Access violation error. The memory at some
address could not be read.  Any access to msg object (of type
RFAMessageWrapper) which is passed from C++ code raises this exception.
It doesn't occurs for basic data types. I wonder what I am missing here.

Regards, Alok

________________________________

From: Jadhav, Alok 
Sent: Monday, December 07, 2009 11:02 AM
To: Brian Cole
Cc: swi...@li...
Subject: Re: [Swig-user] Python C++ extension (multi threaded
environment)

In the example below, the example of int is working fine. The callback
is called properly in Python  and the int value is displayed. All good.

But when I pass the object of type RFAMessageWrapper, even type(msg)
raises an exception. I am not able to send user defined c++ object to
Python. Not sure what is wrong here.

Also the usage of 
 SWIG_PYTHON_THREAD_BEGIN_BLOCK; 
      SWIG_PYTHON_THREAD_END_BLOCK;
doesn't raise any exception. But I wonder if it succeeds in acquiring
the GIL.

Please advice.

Regards, Alok

________________________________

From: Jadhav, Alok 
Sent: Saturday, December 05, 2009 12:32 PM
To: Brian Cole
Cc: swi...@li...
Subject: Re: [Swig-user] Python C++ extension (multi threaded
environment)

I will try to use that but before using the SWIG macros I should be able
to build the arguments list from the C objects. 

arglist = Py_BuildValue("(O)",msg2);             // Build argument list

Above line raises an Access violation exception.
msg2 is non nul C class object.

where as 

int blah =1;
arglist = Py_BuildValue("i",blah);             // Build argument list

this line doesn't raise any exception. 

So it means it has to do with creating a PyObject from C object. (Works
fine for basic datatypes.)

What is the issue in this case?  How can this be solved?

Regards, Alok

________________________________

From: Brian Cole [mailto:co...@ey...] 
Sent: Saturday, December 05, 2009 3:52 AM
To: Jadhav, Alok
Cc: swi...@li...
Subject: Re: [Swig-user] Python C++ extension (multi threaded
environment)

This is a shot in the dark, but have you tried using SWIG's macros for
acquiring and releasing the GIL? 
      SWIG_PYTHON_THREAD_BEGIN_BLOCK;
      SWIG_PYTHON_THREAD_END_BLOCK;

-Brian

On Dec 3, 2009, at 11:57 PM, Jadhav, Alok wrote:

	Guys,

	I created a new wrapper class RFAMesageWrapper which is defined
in SWIG
	interface file.  

	Now the new code looks like

	 RFAMessageWrapper msg2(msg);
	  arglist = Py_BuildValue("(O)",msg2);             // Build
argument
	list

	Where as msg2 is identified by Python.  Still this function
raises
	access violation exception. So my thinking that it was because
	RFAMessage is not identified as RFAMessage was incorrect.

	Now back to square one. . What is wrong here? Why does this line
of code
	raises exception?

	Still couldn't identify why state = PyGILState_Ensure(); blocks
the
	thread.

	Regards,
	Alok

	-----Original Message-----
	From: Jadhav, Alok 
	Sent: Friday, December 04, 2009 3:05 PM
	To: swi...@li...
	Subject: [Swig-user] Python C++ extension (multi threaded
environment)

	Hello,

	I have made some progress in using C library to call from
Python. In
	next step I have to register a callback function so that C can
call
	ProcessMessage() function from which is written in python. My
code is
	not working properly and I have couple of questions regarding
this. 

	Backgroun
	----------

	Python creates RFAClient object. This happens in Python main
thread. Now
	Python calls a function to start postoffice. Postoffice spawns a
new C++
	thread and it runs in a while loop listening to some data. Now
when a
	message arrives on this thread it should call Python function to
process
	this data. Looks simple .. But multi threaded environment, C++
and
	python all together makes it a bit messy :)

	Issues
	------

	My callback registed in C looks like this

	static void PythonCallBack(RFAMessage msg, void *clientdata) {
PyObject
	*func, *arglist;

	PyGILState_STATE state;
	state = PyGILState_Ensure();

	  func = (PyObject *) clientdata;               // Get Python
function
	  arglist = Py_BuildValue("(O)",msg);             // Build
argument
	list
	  PyEval_CallObject(func,arglist);     // Call Python
	  Py_DECREF(arglist);                           // Trash arglist

	PyGILState_Release(state);
	}

	Here 2 lines are giving me exceptions.. (Remember this is called
from
	postoffice thread.. which is not the main Python thread. .. I
have
	called
	PyEval_InitThreads() function; at the start of main thread)

	-> arglist = Py_BuildValue("(O)",msg);
	   - Raises Access Violation error. I have no idea why this is
	occuring. 
	My reasoning : msg is of type RFAMessage.  RFAMessage is not
part of my
	swig interface file, but it is part of the library used by my
C++
	wrapper project. Can RAFMessage object be converted to PyObject
if it is
	not part of SWIG interface file? 
	 What are your suggestions on this?

	-> state = PyGILState_Ensure();
	    The thread just gets blocked on this call. It doesn't do
anything
	after this. After searching on internet I couldn't find anything
which
	can describe why postOffice thread may get blocked here. Any
reasons?
	solutions?

	Please advice. I have spent more than 2 days trying to tackle
this
	single issue.

	Regards,
	Alok
	--
	View this message in context:

http://old.nabble.com/Python-C%2B%2B-extension-%28multi-threaded-environ
	ment%29-tp26635775p26635775.html
	Sent from the swig-user mailing list archive at Nabble.com.

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	a free event focused on virtualization and cloud computing. 
	Attend in-depth sessions from your desk. Your couch. Anywhere.
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	_______________________________________________
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communications disclaimer:
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Re: [Swig-user] Python C++ extension (multi threaded environment) (Somethign useful ? )

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Re: [Swig-user] Python C++ extension (multi threaded environment) (Somethign useful ? )