I declare a C function as Python prototype
static PyObject* MyFunction(PyObject* self, PyObject* args)
{
return Py_None ;
}
Now I want to add it into a dynamically loaded module
PyObject *pymod = PyImport_ImportModule("mymodule");
PyObject_SetAttrString( pymod, "myfunction", ? );
How to convert C function into PyObject callable ?
You need to construct a new PyCFunctionObject object from the MyFunction. Usually this is done under the hood using the module initialization code, but as you're now doing it the opposite way, you need to construct the PyCFunctionObject yourself, using the undocumented PyCFunction_New or PyCFunction_NewEx, and a suitable PyMethodDef:
static PyMethodDef myfunction_def = {
"myfunction",
MyFunction,
METH_VARARGS,
"the doc string for myfunction"
};
...
// Use PyUnicode_FromString in Python 3.
PyObject* module_name = PyString_FromString("mymodule");
if (module_name == NULL) {
// error exit!
}
// this is adapted from code in code in
// Objects/moduleobject.c, for Python 3.3+ and perhaps 2.7
PyObject *func = PyCFunction_NewEx(&myfunction_def, pymod, module_name);
if (func == NULL) {
// error exit!
}
if (PyObject_SetAttrString(module, myfunction_def.ml_name, func) != 0) {
Py_DECREF(func);
// error exit!
}
Py_DECREF(func);
Again, this is not the preferred way to do things; usually a C extension creates concrete module objects (such as _mymodule) and mymodule.py would import _mymodule and put things into proper places.
Related
I'm building C application which will be using Python plugins. When trying to call the method from another Python module, the function PyImport_ImportModule() seems to imports the module properly, then i try to get the function from this module using PyObject_GetAttrString() and all that I get is null.
I already tried using PyModule_GetDict() and PyDict_GetItemString() to get the method from the module, but the effect was the same.
main.c:
#include <stdio.h>
#include <python3.6/Python.h>
int main()
{
PyObject *arg, *pModule, *ret, *pFunc, *pValue, *pMethod, *pDict;
Py_Initialize();
PyObject *sys = PyImport_ImportModule("sys");
PyObject *path = PyObject_GetAttrString(sys, "path");
PyList_Append(path, PyUnicode_FromString("."));
pModule = PyImport_ImportModule("test");
if(pModule == NULL)
{
perror("Can't open module");
}
pMethod = PyObject_GetAttrString(pModule, "myfun");
if(pMethod == NULL)
{
perror("Can't find method");
}
ret = PyEval_CallObject(pMethod, NULL);
if(ret == NULL)
{
perror("Couldn't call method");
}
PyArg_Parse(ret, "&d", pValue);
printf("&d \n", pValue);
Py_Finalize();
return 0;
}
test.py:
def myfun():
c = 123 + 123
print('the result is: ', c)
myfun()
The result i got is:
Can't find method: Success
Segmentation fault (core dumped)
When I used the gdb debugger the output was:
pModule = (PyObject *) 0x7ffff5a96f48
pMethod = (PyObject *) 0x0
Your program is not wroking because the module being imported is the test built-in module, rather than your test.py script. This is because you are appending the current directory to sys.path, so it is checked after every other already existing path in the list. You should insert it at the beginning of the list instead, so that it is checked first.
This will work:
PyObject *sys = PyImport_ImportModule("sys");
PyObject *path = PyObject_GetAttrString(sys, "path");
PyList_Insert(path, 0, PyUnicode_FromString("."));
By the way, you should #include the Python header before anything else, as stated in the documentation:
Note: Since Python may define some pre-processor definitions which affect the standard headers on some systems, you must include Python.h before any standard headers are included.
I embedded the python 3.6 interpreter successfully in a C++ program, but I have a problem.
I'd like to embed two interpreters in the same program:
One which can use my C++ defined module (MyModule)
One which can not use this module.
Regarding the documentation I should call PyImport_AppendInittab before Py_Initialize function, so the module will be available in the whole program, but I'd like to create separate interpreters with separate built-in modules.
Calling Py_Initialize and Py_Finalize doesn't help, the module will be available in the second interpreter, too. By the way calling init and finalize function multiple times causes huge memory leaks, so I think this wouldn't be a good solution even if it would work.
Do you have any idea how to solve this issue?
Full code:
#include <iostream>
#pragma push_macro("_DEBUG")
#undef _DEBUG
#include "Python.h"
#pragma pop_macro("_DEBUG")
static PyObject* Addition (PyObject *self, PyObject *args)
{
double a = 0.0;
double b = 0.0;
if (!PyArg_ParseTuple (args, "dd", &a, &b)) {
return nullptr;
}
double result = a + b;
return PyFloat_FromDouble (result);
}
static PyMethodDef ModuleMethods[] =
{
{"Add", Addition, METH_VARARGS, "Adds numbers."},
{nullptr, nullptr, 0, nullptr}
};
static PyModuleDef ModuleDef = {
PyModuleDef_HEAD_INIT,
"MyModule",
NULL,
-1,
ModuleMethods,
NULL,
NULL,
NULL,
NULL
};
static PyObject* ModuleInitializer (void)
{
return PyModule_Create (&ModuleDef);
}
int main ()
{
Py_SetPythonHome (L".");
Py_SetPath (L"python36.zip\\Lib");
{ // first interpreter
PyImport_AppendInittab ("MyModule", ModuleInitializer);
Py_Initialize ();
PyRun_SimpleString (
"import MyModule\n"
"print (MyModule.Add (1, 2))"
);
Py_Finalize ();
}
{ // second interpreter without AppendInittab (should not find MyModule, but it does)
Py_Initialize ();
PyRun_SimpleString (
"import MyModule\n"
"print (MyModule.Add (1, 2))"
);
Py_Finalize ();
}
system ("pause");
return 0;
}
I am trying to achieve the following: passing a python object to a c++ callback chain (which are typical in many popular c++ libraries). In the c++ code, callbacks pass on objects that have necessary information for consecutive callbacks in the cascade/chain.
Here is a small test code I wrote: we pass a python object to a c routine (case 1) and call it's method. That works ok. But when I pass the python object to a c++ object and try to call it "inside" the c++ object, I get segfault.. :(
Here it goes:
c++ module ("some.cpp"):
#include <stdint.h>
#include <iostream>
#include <Python.h>
/* objective:
* create c++ objects / routines that accept python objects
* then call methods of the python objects inside c++
*
* python objects (including its variables and methods) could be passed along, for example in c++ callback chains ..
* .. and in the end we could call a python callback
*
* Compile and test as follows:
* python setup.py build_ext
* [copy/link some.so where test.py is]
* python test.py
*
*/
class testclass {
public:
testclass(int* i, PyObject* po) {
std::cerr << "testclass constructor! \n";
i=i; po=po;
}
~testclass() {}
void runpo() {
PyObject* name;
const char* mname="testmethod";
name=PyString_FromString(mname);
std::cerr << "about to run the python method .. \n";
PyObject_CallMethodObjArgs(po, name, NULL);
std::cerr << ".. you did it - i will buy you a beer!\n";
}
public:
int* i;
PyObject* po;
};
/* Docstrings */
static char module_docstring[] = "hand-made python module";
/* Available functions */
static PyObject* regi_wrapper(PyObject * self, PyObject * args);
void regi(int* i, PyObject* po);
/* Module specification */
static PyMethodDef module_methods[] = {
{"regi_wrapper",regi_wrapper, METH_VARARGS, "lets see if we can wrap this sucker"},
{NULL, NULL, 0, NULL}
};
/* Initialize the module */
PyMODINIT_FUNC initsome(void)
{
PyObject *m = Py_InitModule3("some", module_methods, module_docstring);
if (m == NULL)
return;
// import_array(); // numpy not required here ..
}
static PyObject* regi_wrapper(PyObject * self, PyObject * args)
{
int* input_i; // whatever input variable
PyObject* input_po; // python object
PyObject* ret; // return variable
// parse arguments
if (!PyArg_ParseTuple(args, "iO", &input_i, &input_po)) {
return NULL;
}
// https://stackoverflow.com/questions/16606872/calling-python-method-from-c-or-c-callback
// Py_INCREF(input_po); // need this, right? .. makes no difference
/* // seems not to make any difference ..
PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
*/
regi(input_i, input_po);
// PyGILState_Release(gstate); // .. makes no difference
// Py_DECREF(input_po); // .. makes no difference
Py_RETURN_TRUE;
}
void regi(int* i, PyObject* po) {
// search variable and methods from PyObject "po" and call its methods?
PyObject* name;
const char* mname="testmethod";
testclass* testobj;
testobj=new testclass(i,po);
/* [insert // in front of this line to test case 1]
// ***** this one works! *********
name=PyString_FromString(mname);
PyObject_CallMethodObjArgs(po, name, NULL);
*/ // [insert // in front of this line to test case 1]
// *** I WOULD LIKE THIS TO WORK *** but it gives segfault.. :(
testobj->runpo(); // [uncomment to test case 2]
}
setup.py:
from distutils.core import setup, Extension
# the c++ extension module
extension_mod = Extension("some", ["some.cpp"])
setup(name = "some", ext_modules=[extension_mod])
test.py:
import some
class sentinel:
def __init__(self):
pass
def testmethod(self):
print "hello from sentinel.testmethod"
pass
se=sentinel()
some.regi_wrapper(1,se)
This question seems relevant:
Calling python method from C++ (or C) callback
.. however the answer did not help me.
What am I missing/misunderstanding here (my c++ sucks big time, so I might have missed something obvious) .. ?
Also, some bonus questions:
a) I am familiar with swig and swig "directors".. however, I would like to use swig for general wrapping of the code, but my custom wrapping for the sort of things described in this question (i.e. without directors). Is there any way to achieve this?
b) Any other suggestions to achieve what I am trying to achieve here, are highly appreciated.. is this possible or just pure insanity?
Using in the constructor
po=this->po
solves the "issue". Sorry for the spam! I will leave here this thing as an example.. maybe someone finds it useful.
In an attempt to call a c function from Python, (in a previous post Calling a C function from a Python file. Getting error when using Setup.py file), I have compiled the code into a .pyd file and am testing the program.
However, I am coming across the error
AttributeError: 'module' object has no attribute 'addTwo'
My test file is as so:
import callingPy
a = 3
b = 4
s = callingPy.addTwo(a, b)
print("S", s)
Where callingPy is the following .c file (turned into a .pyd) through compilation:
#include <Python.h>
#include "adder.h"
static PyObject* adder(PyObject *self, PyObject *args)
{
int a;
int b;
int s;
if (!PyArg_ParseTuple(args,"ii",&a,&b))
return NULL;
s = addTwo(a,b);
return Py_BuildValue("i",s);
}
/* DECLARATION OF METHODS*/
static PyMethodDef ModMethods[] = {
{"modsum", adder, METH_VARARGS, "Descirption"},
{NULL,NULL,0,NULL}
};
// Module Definition Structure
static struct PyModuleDef summodule = {
PyModuleDef_HEAD_INIT,"modsum", NULL, -1, ModMethods
};
/* INITIALIZATION FUNCTION*/
PyMODINIT_FUNC PyInit_callingPy(void)
{
PyObject *m;
m = PyModule_Create(&summodule);
return m;
}
Any help would be greatly appreciated!
Thank you.
The only function in the extension module is exported to Python under the name modsum. You called addTwo. This seems self-explanatory.
It looks like at the C layer, there is a raw C function named addTwo that does the work for the C function adder, which is then exported to Python under the name modsum. So you should either rename the export, or call it with the correct name:
s = callingPy.modsum(a, b)
It looks like you copy-pasted a skeleton extension module, switched one tiny internal, and didn't fix up any of the exports or names.
Sorry, there's a lot here, but I really think all of this information is relevant to the question I'm about to ask.
So I wrapped my C++ class using SWIG, compiled, etc. and in a python script I create an instance of my class and then try to call a method on that class. Notice I said "try" to call. The call on the method fails. Details on implementation & "fails" follows.
First, I am building the Python3.4 interpreter with my Windows application and calling my python script as follows:
Py_Initialize();
/* Some code here to set up python paths. */
.....
PyObject *pName = PyUnicode_FromString("hello");
if(pName == NULL)
{
Py_Finalize();
return false;
}
PyObject* pModule = PyImport_Import(pName);
if(pModule == NULL)
{
Py_Finalize();
return false;
}
/* "go" is the name of the function in my python script that I want to call. */
PyObject *pFunc = PyObject_GetAttrString(pModule, "go");
if(pFunc == NULL)
{
Py_Finalize();
return false;
}
PyObject *pArgs = PyTuple_New(0);
PyObject *pValue = PyObject_CallObject(pFunc, pArgs);
if(pValue == NULL)
{
Py_Finalize();
return false;
}
Py_Finalize();
return true;
===================
Here's my class structure:
class BoganDocument
{
private:
BoganMetadataSet myMetadata;
public:
// Constructor
BoganDocument();
// Destructor
virtual ~BoganDocument();
// Useful methods.
wstring getMetadataValue(wstring metadata_name);
}
===================
Here's my Python script (named "hello.py").
import BoganDocument
def go():
print("I'm in go()")
d = BoganDocument.BoganDocument()
print("I made a document")
ts = d.getMetadataValue("CreationTimestamp");
print("The creation timestamp is " + ts)
=====================
And the output on my console screen looks like this:
trying C:\Program Files (x86)\MyApp\Python\swig_runtime_data4_d.pyd
trying C:\Program Files (x86)\MyApp\Python\swig_runtime_data4.py
trying C:\Program Files (x86)\MyApp\Python\swig_runtime_data4.pyw
trying C:\Program Files (x86)\MyApp\Python\swig_runtime_data4.pyc
trying c:\MyApp\Workplace\swig_runtime_data4_d.pyd
trying c:\MyApp\Workplace\swig_runtime_data4.py
trying c:\MyApp\Workplace\swig_runtime_data4.pyw
trying c:\MyApp\Workplace\swig_runtime_data4.pyc
import 'BoganDocument' # <_frozen_importlib.SourceFileLoader object at 0x10D2E3F0>
import 'hello' # <_frozen_importlib.SourceFileLoader object at 0x10D2E1C0>
I'm in go()
I made a document
===================
Notice that I have set PYTHONVERBOSE to 11 to get as much diagnostic information as I can. And notice that all messages stop after "print("I made a document")". No syntax error, nothing. The value of pValue after "PyObject *pValue = PyObject_CallObject(pFunc, pArgs);" is NULL.
Now, the call to the constructor at "d = BoganDocument.BoganDocument()" is known to work, as I've set a breakpoint in the BoganDocument constructor and single stepped through.
I also set a breakbpoint in BoganDocument's getMetadataValue method but never get there. And there's the mystery.
No doubt I have a bug, but I haven't a clue.
Did you step into the SWIG wrapper for getMetadataValue? The C++ wrapper gets called when you do d.getMetadataValue() frm Python, the C++ wrapper then calls Bogan::getMetadataValue on the appropriate C++ instance. Since you never get to the C++ instance's getMetadataValue, maybe SWIG wrapper is finding the call to be illegal or such (SWIG doesn't normally silently ignore problems but I don't know what else to suggest). Since you are using wide string, make sure you include std_wstring.i.