How to transfer data structure or class from Python to C/C++?

H

Hongtian

Hi friends,

I am a newer of Python. I want to ask below question:

I have a C/C++ application and I want to use Python as its extension.
To do that, I have to transfer some data structure from C/C++
application to Python and get some data structure from Python to C/C++
application. I have researched Python document, but the example only
describes how to transfer some simple data, such as integer,char,
etc.

Could you please guide me to do that? or tell me some document to have
a research?

Thanks.
 
B

bearophileHUGS

Hongtian:
Could you please guide me to do that? or tell me some document to have
a research?
Click to expand...

You can start googling for:
- SWIG
- Boost.Python
- SIP
- ctypes (built-in module)
- And more.

Bye,
bearophile
 
A

Aaron \Castironpi\ Brady

Hi friends,

I am a newer of Python. I want to ask below question:

I have a C/C++ application and I want to use Python as its extension.
To do that, I have to transfer some data structure from C/C++
application to Python and get some data structure from Python to C/C++
application. I have researched Python document, but the example only
describes how to transfer some simple data, such as integer,char,
etc.

Could you please guide me to do that? or tell me some document to have
a research?

Thanks.
Click to expand...

Take this for what it's worth. If I understand correctly, you want
this:

struct info {
char* name;
char* address;
int age;
};

int main( ) {
info A, B;
python_run( "\
from urllib import urlget\n\
from mylib import populate_struct\n\
page= urlget( 'http://something' )\n\
populate_struct( page, A )\n\
populate_struct( page, B )\n" );
if( A.age> B.age ) {
something_in_C( );
}
return 0;
}

Am I on the right track? Do you have any questions so far?
 
H

Hongtian

Not exactly.

In my C/C++ application, I have following function or flow:

void func1(....)
{
call PyFunc(struct Tdemo, struct &Tdemo1);
}

I mean I want to invoke Python function 'PyFunc' and transfer a data
structure 'Tdemo' to this function. After some process in Python, I
want it return 'Tdemo1' back to the C/C++ application.

I research boost.python and think it is not a reasonable solution
because it make the C/C++ application too complex.

Thanks.
 
A

Aaron \Castironpi\ Brady

Not exactly.

In my C/C++ application, I have following function or flow:

void func1(....)
{
    call PyFunc(struct Tdemo, struct &Tdemo1);

}

I mean I want to invoke Python function 'PyFunc' and transfer a data
structure 'Tdemo' to this function. After some process in Python, I
want it return 'Tdemo1' back to the C/C++ application.

I research boost.python and think it is not a reasonable solution
because it make the C/C++ application too complex.

Thanks.
Click to expand...

I am stumped. Here's what I have.

/C file:

typedef struct {
int a;
float b;
} TypeA;

static PyObject *
methA(PyObject *self, PyObject *args) {
TypeA a;
TypeA b;
PyObject* fun;
PyObject* res;
TypeA* pa;
TypeA* pb;

PyArg_ParseTuple( args, "O", &fun );
a.a= 10;
a.b= 20.5;
b.a= 30;
b.b= 40.5;
printf( "%p %p\n", &a, &b );

pa= &a;
pb= &b;

res= PyObject_CallFunction( fun, "II", &pa, &pb );
Py_DECREF( res );

return PyInt_FromLong( 0 );
}

/Py file:

import ng27ext

import ctypes as c
class TypeA( c.Structure ):
_fields_= [
( 'a', c.c_int ),
( 'b', c.c_float )
]

def exposed( obj1, obj2 ):
print 'in exposed'
print hex( obj1 ), hex( obj2 )
a= c.POINTER( TypeA ).from_address( obj1 )
print a
print a.contents

print ng27ext.methA( exposed )

/Output:

0021FD48 0021FD40
in exposed
0x21fd48 0x21fd40
<ctypes.LP_TypeA object at 0x009FF350>
<__main__.TypeA object at 0x009FF4E0>
0

Which is unexpected. The address on line 4 should be the contents of
'obj1', 0x21fd48, which it's not. I must not be using 'from_address'
properly.
 
A

Aaron \Castironpi\ Brady

Not exactly.

In my C/C++ application, I have following function or flow:

void func1(....)
{
    call PyFunc(struct Tdemo, struct &Tdemo1);

}

I mean I want to invoke Python function 'PyFunc' and transfer a data
structure 'Tdemo' to this function. After some process in Python, I
want it return 'Tdemo1' back to the C/C++ application.

I research boost.python and think it is not a reasonable solution
because it make the C/C++ application too complex.

Thanks.
Click to expand...

Did you have any luck?
 
A

Aaron \Castironpi\ Brady

Not exactly.

In my C/C++ application, I have following function or flow:

void func1(....)
{
call PyFunc(struct Tdemo, struct &Tdemo1);

}

I mean I want to invoke Python function 'PyFunc' and transfer a data
structure 'Tdemo' to this function. After some process in Python, I
want it return 'Tdemo1' back to the C/C++ application.

I research boost.python and think it is not a reasonable solution
because it make the C/C++ application too complex.

Thanks.
Click to expand...

Did you have any luck?
 
A

Aaron \Castironpi\ Brady

Not exactly.

In my C/C++ application, I have following function or flow:

void func1(....)
{
    call PyFunc(struct Tdemo, struct &Tdemo1);

}

I mean I want to invoke Python function 'PyFunc' and transfer a data
structure 'Tdemo' to this function. After some process in Python, I
want it return 'Tdemo1' back to the C/C++ application.

I research boost.python and think it is not a reasonable solution
because it make the C/C++ application too complex.

Thanks.
Click to expand...
snip

Solution produced here. Includes dirty kludge, which will welcome
correction.

/C file:

#include <Python.h>

typedef struct {
int a;
float b;
} TypeA;

static PyObject *
methA(PyObject *self, PyObject *args) {
TypeA a;
TypeA b;
PyObject* fun;
PyObject* res;

PyArg_ParseTuple( args, "O", &fun );
a.a= 10;
a.b= 20.5;

res= PyObject_CallFunction( fun, "II", &a, &b );

printf( "%i %f\n", b.a, b.b );
Py_DECREF( res );

return PyInt_FromLong( 0 );
}

static PyMethodDef module_methods[] = {
{"methA", methA, METH_VARARGS, "No doc"},
{NULL, NULL, 0, NULL} /* Sentinel */
};


#ifndef PyMODINIT_FUNC /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
initng27ext(void)
{
PyObject* m;
m = Py_InitModule3("ng27ext", module_methods,
"Custom.");
if (m == NULL)
return;
}

/Py file:

import ng27ext

import ctypes as c
class TypeA( c.Structure ):
_fields_= [
( 'a', c.c_int ),
( 'b', c.c_float )
]

from _ctypes import _cast_addr
_data_cast= c.PYFUNCTYPE( c.py_object, c.c_void_p, c.py_object,
c.py_object)( _cast_addr ) #dirty kludge

def exposed( obj1, obj2 ):
cob1= _data_cast( obj1, None, c.POINTER( TypeA ) )
cob2= _data_cast( obj2, None, c.POINTER( TypeA ) )
print cob1.contents.a, cob1.contents.b
cob2.contents.a= c.c_int( 60 )
cob2.contents.b= c.c_float( 70.5 )
print cob2.contents.a, cob2.contents.b

print ng27ext.methA( exposed )

/Compile & link:

c:/programs/mingw/bin/gcc ng27ext.c -c -Ic:/programs/python25/include
c:/programs/mingw/bin/gcc -shared ng27ext.o -o ng27ext.pyd -Lc:/
programs/python25/libs -lpython25

/Output:

10 20.5
60 70.5
60 70.500000
0
Press any key to continue . . .
 
G

Gabriel Genellina

En Fri, 17 Oct 2008 20:03:44 -0300, Aaron "Castironpi" Brady
Not exactly.

In my C/C++ application, I have following function or flow:

void func1(....)
{
    call PyFunc(struct Tdemo, struct &Tdemo1);

}

I mean I want to invoke Python function 'PyFunc' and transfer a data
structure 'Tdemo' to this function. After some process in Python, I
want it return 'Tdemo1' back to the C/C++ application.

I research boost.python and think it is not a reasonable solution
because it make the C/C++ application too complex.

Thanks.
Click to expand...
snip

Solution produced here. Includes dirty kludge, which will welcome
correction.

/C file:

#include <Python.h>

typedef struct {
int a;
float b;
} TypeA;

static PyObject *
methA(PyObject *self, PyObject *args) {
TypeA a;
TypeA b;
PyObject* fun;
PyObject* res;

PyArg_ParseTuple( args, "O", &fun );
a.a= 10;
a.b= 20.5;

res= PyObject_CallFunction( fun, "II", &a, &b );

printf( "%i %f\n", b.a, b.b );
Py_DECREF( res );

return PyInt_FromLong( 0 );
}

static PyMethodDef module_methods[] = {
{"methA", methA, METH_VARARGS, "No doc"},
{NULL, NULL, 0, NULL} /* Sentinel */
};


#ifndef PyMODINIT_FUNC /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
initng27ext(void)
{
PyObject* m;
m = Py_InitModule3("ng27ext", module_methods,
"Custom.");
if (m == NULL)
return;
}

/Py file:

import ng27ext

import ctypes as c
class TypeA( c.Structure ):
_fields_= [
( 'a', c.c_int ),
( 'b', c.c_float )
]

from _ctypes import _cast_addr
_data_cast= c.PYFUNCTYPE( c.py_object, c.c_void_p, c.py_object,
c.py_object)( _cast_addr ) #dirty kludge

def exposed( obj1, obj2 ):
cob1= _data_cast( obj1, None, c.POINTER( TypeA ) )
cob2= _data_cast( obj2, None, c.POINTER( TypeA ) )
print cob1.contents.a, cob1.contents.b
cob2.contents.a= c.c_int( 60 )
cob2.contents.b= c.c_float( 70.5 )
print cob2.contents.a, cob2.contents.b

print ng27ext.methA( exposed )

/Compile & link:

c:/programs/mingw/bin/gcc ng27ext.c -c -Ic:/programs/python25/include
c:/programs/mingw/bin/gcc -shared ng27ext.o -o ng27ext.pyd -Lc:/
programs/python25/libs -lpython25

/Output:

10 20.5
60 70.5
60 70.500000
0
Press any key to continue . . .
Click to expand...
 

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