how to pass C++ object to another C++ function via Python function

[grbgooglefan]

I am trying to pass a C++ object to Python function. This Python
function then calls another C++ function which then uses this C++
object to call methods of that object's class.

I tried something like this, but it did not work, gave core dump.

class myclass {
public:
myclass(){};
~myclass(){};
void printmyname() { printf("I am myclass, num=%d\n",num); };
};

main(){
myclass myobj

char funcbody[]=\
"def pyfunction(t167 ,classobj):\n\
onemorefunc(t16 7,\"NONAMe\")\n \
return 10\n\
\n\n";

// compile this Python function using PyRun_String & get its pointer
by PyObject_GetAtt rString.
// Later call it as below:
myclass myobj(23);
PyObject *pTuple = 0;
pTuple = PyTuple_New(2);
PyTuple_SetItem (pTuple,0,PyStr ing_FromString( "NAME")); // for t167
parameter
PyObject *inputarg = Py_BuildValue(" OO&",pTuple,myo bj); // for
classobj parameter

result = PyObject_CallOb ject(pPyEvalFun ction,inputarg) ;
}

How can I pass this class object to Python function?
Is it possible to set it in tuple using PyTuple_SetItem , because I may
have varying number of arguments for my Python functions & that's why
I can't use Py_BuildValue.

[Diez B. Roggisch]

grbgooglefan wrote:

I am trying to pass a C++ object to Python function. This Python
function then calls another C++ function which then uses this C++
object to call methods of that object's class.

You might consider using a C++-wrapper like SIP, Swig or Boost::Python to do
this.

If you don't like that, all I can think of would be to return the address of
the object as integer, and pass that around. Then in the appropriate
C++-call, cast that integer to the object. butt-ugly and -10 style-points
though.

Diez

[Gabriel Genellina]

En Mon, 21 Apr 2008 10:24:15 -0300, grbgooglefan <ga*********@gm ail.comescribió :

I am trying to pass a C++ object to Python function. This Python
function then calls another C++ function which then uses this C++
object to call methods of that object's class.

I tried something like this, but it did not work, gave core dump.

You can't pass any arbitrary C object to a Python function.
In this case you can use a PyCObject, a Python box around a void* pointer.
See http://docs.python.org/api/cObjects.html

// compile this Python function using PyRun_String & get its pointer
by PyObject_GetAtt rString.
// Later call it as below:
myclass myobj(23);
PyObject *pTuple = 0;
pTuple = PyTuple_New(2);
PyTuple_SetItem (pTuple,0,PyStr ing_FromString( "NAME")); // for t167
parameter
PyObject *inputarg = Py_BuildValue(" OO&",pTuple,myo bj); // for
classobj parameter

result = PyObject_CallOb ject(pPyEvalFun ction,inputarg) ;
}

How can I pass this class object to Python function?
Is it possible to set it in tuple using PyTuple_SetItem , because I may
have varying number of arguments for my Python functions & that's why
I can't use Py_BuildValue.

You have to check every call for errors, and pay attention to the reference counts!
The argument passing is wrong. You never set the second tuple element. An easier way is using PyObject_CallFu nctionObjArgs(f unction, arg1, arg2, NULL)

--
Gabriel Genellina

[Diez B. Roggisch]

Gabriel Genellina wrote:

En Mon, 21 Apr 2008 10:24:15 -0300, grbgooglefan <ga*********@gm ail.com>
escribió:

>I am trying to pass a C++ object to Python function. This Python
function then calls another C++ function which then uses this C++
object to call methods of that object's class.

I tried something like this, but it did not work, gave core dump.

You can't pass any arbitrary C object to a Python function.
In this case you can use a PyCObject, a Python box around a void* pointer.
See http://docs.python.org/api/cObjects.html

Neat! Didn't know about that one.

Diez

[grbgooglefan]

On Apr 21, 10:17*pm, "Gabriel Genellina" <[e-mail].a r>
wrote:

En Mon, 21 Apr 2008 10:24:15 -0300, grbgooglefan <ganeshbo...@gm ail.comescribió :

I am trying to pass a C++ object to Python function. This Python
function then calls another C++ function which then uses this C++
object to call methods of that object's class.

I tried something like this, but it did not work, gave core dump.

You can't pass any arbitrary C object to a Python function.
In this case you can use a PyCObject, a Python box around a void* pointer.
Seehttp://docs.python.org/api/cObjects.html

Yup, I looked at http://www.python.org/doc/ext/using-cobjects.html
also. I could not find in this example where is CObject used or
converted back from Python to C.
Is there any tutorial which I can use for this?

[Gabriel Genellina]

En Mon, 21 Apr 2008 19:11:31 -0300, grbgooglefan <ga*********@gm ail.com>
escribió:

On Apr 21, 10:17*pm, "Gabriel Genellina" <[e-mail].a r>
wrote:

>En Mon, 21 Apr 2008 10:24:15 -0300, grbgooglefan
<ganeshbo...@g mail.comescribi ó:

I am trying to pass a C++ object to Python function. This Python
function then calls another C++ function which then uses this C++
object to call methods of that object's class.

I tried something like this, but it did not work, gave core dump.

You can't pass any arbitrary C object to a Python function.
In this case you can use a PyCObject, a Python box around a void*
pointer.
Seehttp://docs.python.org/api/cObjects.html

Yup, I looked at http://www.python.org/doc/ext/using-cobjects.html
also. I could not find in this example where is CObject used or
converted back from Python to C.
Is there any tutorial which I can use for this?

If you have a C function that receives a PyCObject, just include the
relevant headers (cobject.h) and you can retrieve the original pointer
using PyCObject_AsVoi dPtr:

void foo(PyObject *pyobj)
{
TOriginalType *porig;
porig = (TOriginalType *)PyCObject_AsV oidPtr(pyobj);
// do something with porig
}

--
Gabriel Genellina

[Gabriel Genellina]

En Mon, 21 Apr 2008 11:19:24 -0300, Diez B. Roggisch <de***@nospam.w eb.de>
escribió:

Gabriel Genellina wrote:

>You can't pass any arbitrary C object to a Python function.
In this case you can use a PyCObject, a Python box around a void*
pointer.
See http://docs.python.org/api/cObjects.html

Neat! Didn't know about that one.

I found it just by chance. The title alone in the API reference isn't very
descriptive...

--
Gabriel Genellina

[grbgooglefan]

On Apr 22, 7:54*am, "Gabriel Genellina" <[e-mail].a r>
wrote:

>
If you have a C function that receives a PyCObject, just include the *
relevant headers (cobject.h) and you can retrieve the original pointer *
using PyCObject_AsVoi dPtr:

void foo(PyObject *pyobj)
{
* *TOriginalType *porig;
* *porig = (TOriginalType *)PyCObject_AsV oidPtr(pyobj);
* *// do something with porig
}
--
Gabriel Genellina- Hide quoted text -

- Show quoted text -

This works. But only once, means if I try to use this object second
time in Python function it causes crash.

What I am doing in my program is that I am putting STL map in a
structure & passing that structure as object to a Python function
alongwith other agurments of that Python function. This briefly as
below:

// In pyinterface.h file:---
typedef hash_map<char*, intElements;
typedef hash_map<char*, Elements*,Strin gHash,eqstr>
PerExchGroupsEl ementsTable;
typedef struct capsule {
PerExchGroupsEl ementsTable* refgrps;
} *pcapsule;

// In pyinterface.cpp file:---
numvars = // value set depending on number of variables of that python
function
PyObject *pTuple = PyTuple_New(num vars);

// Set variables as below:
for(nCtr = 0; nCtr < numvars; nCtr++){
slot = prul->pVarLst[nCtr].slot;
ndtyp = ordvars[slot].dtype;
switch(ndtyp){
case(INT_T):

PyTuple_SetItem (pTuple,nCtr,Py Int_FromLong(or dvars[slot].nvalue));
break;
case(FLOAT_T):

PyTuple_SetItem (pTuple,nCtr,Py Float_FromDoubl e(ordvars[slot].fvalue));
break;
case(STRING_T):

PyTuple_SetItem (pTuple,nCtr,Py String_FromStri ng(ordvars[slot].cvalue));
break;
default:
printf("\nUnkno wn data type [%d] for %s\n",ndtyp,pru l-

>pVarLst[nCtr].var);

bUnknownDataTyp e = true;
break;
}
if(bUnknownData Type){
ret = -1;
break;
}
}

// Then set the C++ object as below:
if(ret == 0){
capsule grpob;
if(pGroups){
grpob.refgrps = pGroups; // pGroups is pointer to
PerExchGroupsEl ementsTable map & is global.
int ret = PyTuple_SetItem (pTuple,
(numvars-1),PyCObject_Fr omVoidPtr((void *)&grpob, NULL));
}
PyObject *pResult = PyObject_CallOb ject(pfunc,pTup le);
if(PyErr_Occurr ed()){
printf("error occured in PyObject_CallOb ject for %s\n",prul-

>pyobj.szPyRout eName);

PyErr_Print();
} else {
printf("Python function passed, use its result for other
purposes as designed\n");
}
Py_XDECREF(pRes ult);
Py_XDECREF(pTup le);
}

//---------- My Pythong module & functions in it -------------------
//PyObject* pfunc is a Python function which I compile dynamically &
add to my Python module as below:
// One of such dynamically compiled function is as below:
char pyfunction[]=\
"def TSE581(t22,t52, t1012,ob):
if(co1(ob,t22,\ "TSE_FUTURE_GRP \") and
like1(ob,t52,\" TSE_SECID_LST2\ ")):\n\
print \"result is pass\"\n\
return 1\n\
else:\n\
print \"result is fail\"\n\
return 0\n";

// function parameter "ob" in this function definition is the one
which Im passing as CObject.

PyObject *result = NULL;
result =
PyRun_String(py function,Py_fil e_input,_pPyDic tionary,_pPyDic tionary);
if(PyErr_Occurr ed() || result == NULL){
printf("Failed to compile function [%s]\n",func);
PyErr_Print();
return;
}
Py_XDECREF(resu lt);
result = NULL;
PyObject *ptr = PyObject_GetAtt rString(_pPyMod ule,fname);
if(PyErr_Occurr ed() || *ptr == NULL){
printf("PyObjec t_GetAttrString failed:%s",fnam e);
return;
}
if(!PyCallable_ Check(*ptr)){
printf("%s not a callable Python code\n",fname);
Py_XDECREF(*ptr );
*ptr = NULL;
return;
}

// I've created dynamically loadble Python module & multiple functions
similar to above gets added dynamically to this module.
// Module has functions "co1" & "like1" in module's .cpp file. These
functions then use CObject - the struct capsule & map pointer in it.
static PyObject* merorderrouter_ co1(PyObject* self, PyObject* args)
{
printf("Contain s function\n");
int ret=0;
char *arg1=NULL, *arg2=NULL;
PyObject* voidcap=NULL;
if(!PyArg_Parse Tuple(args, "Oss", &voidcap,&arg1, &arg2)){
printf("failed to get args\n");
if(PyErr_Occurr ed())
PyErr_Print();
return(PyInt_Fr omLong(ret));
}
printf("key=%s, grpname=[%s]\n",arg1,arg2 );
if(voidcap && PyCObject_Check (voidcap))
printf("valid Py-C-Object\n");
else
printf("NOT a valid Py-C-Object\n");
pcapsule pobj = (pcapsule)PyCOb ject_AsVoidPtr( voidcap);
if(pobj){
PerExchGroupsEl ementsTable grpmap = *pobj->refgrps;
if(grpmap.count (arg2)){
PerExchGroupsEl ementsTable::it erator grpmi =
grpmap.find(arg 2);
Elements grpelems = *(Elements*)grp mi->second;
Elements::itera tor ei = grpelems.find(a rg1);
if(ei != grpelems.end()) {
printf("has elm.\n");
ret=1;
} else {
printf("no elm.\n");
ret=0;
}
}
} else {
printf("pcapsul e object is null from PyCObject_AsVoi dPtr
\n");
if(PyErr_Occurr ed())
PyErr_Print();
}
printf("------- co returning.....\ n");
return PyInt_FromLong( ret);
}
//=============== =============== =============== ==========

What happens is that when Python function TSE581 gets called from my C
Program via PyObject_CallOb ject (as shown at top of this post), co1
function works fine & can access the map pointer properly.
But when next function like1 gets called, crash happens.

My queries are & things on which I need help are:
1) Is there anything wrong I am doing when I am passing the C-Object
from my C++ code->Python -C++ code again?
2) Am I missing to increase or decrease the reference count somewhere.
3) I dont want map pointer to be ever freed because it is Process
level data structure & requried at every execution of these Python
functions. How do I avoid its cleanup when it gets passed to Python &
Python cleans up those objects.

If want to see my Python module & the CPP code using it, I can send
all my source code to you.
Please help.

[Gabriel Genellina]

En Fri, 02 May 2008 00:26:38 -0300, grbgooglefan <ga*********@gm ail.com>
escribió:

On Apr 22, 7:54*am, "Gabriel Genellina" <[e-mail].a r>
wrote:

>>
If you have a C function that receives a PyCObject, just include the *
relevant headers (cobject.h) and you can retrieve the original pointer *
using PyCObject_AsVoi dPtr:

This works. But only once, means if I try to use this object second
time in Python function it causes crash.

What I am doing in my program is that I am putting STL map in a
structure & passing that structure as object to a Python function
alongwith other agurments of that Python function. This briefly as
below:

// In pyinterface.h file:---
typedef hash_map<char*, intElements;
typedef hash_map<char*, Elements*,Strin gHash,eqstr>
PerExchGroupsEl ementsTable;
typedef struct capsule {
PerExchGroupsEl ementsTable* refgrps;
} *pcapsule;

// In pyinterface.cpp file:---
numvars = // value set depending on number of variables of that python
function
PyObject *pTuple = PyTuple_New(num vars);

// Set variables as below:
for(nCtr = 0; nCtr < numvars; nCtr++){
slot = prul->pVarLst[nCtr].slot;
ndtyp = ordvars[slot].dtype;
switch(ndtyp){
case(INT_T):
PyTuple_SetItem (pTuple,nCtr,Py Int_FromLong(or dvars[slot].nvalue));
break;
case(FLOAT_T):
PyTuple_SetItem (pTuple,nCtr,Py Float_FromDoubl e(ordvars[slot].fvalue));
break;
case(STRING_T):
PyTuple_SetItem (pTuple,nCtr,Py String_FromStri ng(ordvars[slot].cvalue));
break;
default:
printf("\nUnkno wn data type [%d] for %s\n",ndtyp,pru l-

>pVarLst[nCtr].var);

bUnknownDataTyp e = true;
break;
}
if(bUnknownData Type){
ret = -1;
break;
}
}

// Then set the C++ object as below:
if(ret == 0){
capsule grpob;
if(pGroups){
grpob.refgrps = pGroups; // pGroups is pointer to
PerExchGroupsEl ementsTable map & is global.
int ret = PyTuple_SetItem (pTuple,
(numvars-1),PyCObject_Fr omVoidPtr((void *)&grpob, NULL));
}

This look suspicious - what if !pGroups? You can't leave a tuple item
uninitialized - if you create a tuple with PyTuple_New(3), then you have
to fill the 3 items.
And beware of that local variable grpob - you're using a pointer to it,
and it won't be valid when execution goes out of this block. You must
ensure that nobody stores a reference to it.
And you should decref the tuple even if this block isn't executed.

PyObject *ptr = PyObject_GetAtt rString(_pPyMod ule,fname);
if(PyErr_Occurr ed() || *ptr == NULL){
printf("PyObjec t_GetAttrString failed:%s",fnam e);
return;
}
if(!PyCallable_ Check(*ptr)){
printf("%s not a callable Python code\n",fname);
Py_XDECREF(*ptr );
*ptr = NULL;
return;
}

All those *ptr should be ptr (the compiler should issue a lot of warnings,
I presume?)

What happens is that when Python function TSE581 gets called from my C
Program via PyObject_CallOb ject (as shown at top of this post), co1
function works fine & can access the map pointer properly.
But when next function like1 gets called, crash happens.

My queries are & things on which I need help are:
1) Is there anything wrong I am doing when I am passing the C-Object
from my C++ code->Python -C++ code again?

I see nothing obviously wrong, except the above notes. If you get warnings
from the C++ compiler, try to fix all of them.

2) Am I missing to increase or decrease the reference count somewhere.

In case of error you exit early but without releasing some existing
objects. (Sometimes a goto statement *is* the right thing to do)

3) I dont want map pointer to be ever freed because it is Process
level data structure & requried at every execution of these Python
functions. How do I avoid its cleanup when it gets passed to Python &
Python cleans up those objects.

Python will do nothing with the pointer inside a PyCObject - unless you
want to, and pass a cleanup function as the second argument to the
PyCObject constructor.

--
Gabriel Genellina

[grbgooglefan]

Yes, that worked. I "protected" the variable which I did not want to
get freed. I incremented reference for that variable & it started
working.

Thanks for all your guidance.

On May 3, 5:23*am, "Gabriel Genellina" <[e-mail].a rwrote:

Python will do nothing with the pointer inside a PyCObject - unless you *
want to, and pass a cleanup function as the second argument to the *
PyCObject constructor.

--
Gabriel Genellina- Hide quoted text -

- Show quoted text -