How To catch python stdout in c++ code
I have a program which during it's run sometimes needs to call python in order to preform some tasks. I need a function that calls python and catches pythons stdout and puts it in some file. This is a declaration of the function
pythonCallBackFunc(const char* pythonInput)
My problem is to catch all the python output for a given command (pythonInput). I have no experience with python API and I don't know what is the right technique to do this. First thing I've tried is to redirect python's sdtout and stderr using Py_run_SimpleString this is some example of the code i've written.
#include "boost\python.hpp"
#include <iostream>
void pythonCallBackFunc(const char* inputStr){
PyRun_SimpleString(inputStr);
}
int main () {
...
//S0me outside functions does this
Py_Initialize();
PyRun_SimpleString("import sys");
PyRun_SimpleString("old_stdout = sys.stdout");
PyRun_SimpleString("fsock = open('python_out.log','a')");
PyRun_SimpleString("sys.stdout = fsock");
...
//my func
pythonCallBackFunc("print 'HAHAHAHAHA'");
pythonCallBackFunc("result = 5");
pythonCallBackFunc("print result");
pythonCallBackFunc("result = 'Hello '+'World!'");
pythonCallBackFunc("print result");
pythonCallBackFunc("'KUKU '+'KAKA'");
pythonCallBackFunc("5**3");
pythonCallBackFunc("prinhghult");
pythonCallBackFunc("execfile('stdout_close.py')");
...
//Again anothers function code
PyRun_SimpleString("sys.stdout = old_stdout");
PyRun_SimpleString("fsock.close()");
Py_Finalize();
return 0;
}
Is there a better way to do this? Besides, for some reason PyRun_SimpleString does nothing when it gets some mathematical expression, for example PyRun_SimpleString("5**3") prints nothing (python conlsul prints the result: 125)
maybe it is important, i am using visual studio 2008. Thanks, Alex
Changes I've made according Mark's suggestion:
#include <python.h>
#include <string>
using namespace std;
void PythonPrinting(string inputStr){
string stdOutErr =
"import sys\n\
class CatchOut:\n\
def __init__(self):\n\
self.value = ''\n\
def write(self, txt):\n\
self.value += txt\n\
catchOut = CatchOut()\n\
sys.stdout = catchOut\n\
sys.stderr = catchOut\n\
"; //this is python code to redirect stdouts/stderr
PyObject *pModule = PyImport_AddModule("__main__"); //create main module
PyRun_SimpleString(stdOutErr.c_str()); //invoke code to redirect
PyRun_SimpleString(inputStr.c_str());
PyObject *catcher = PyObject_GetAttrString(pModule,"catchOut");
PyObject *output = PyObject_GetAttrString(catcher,"value");
printf("Here's the output: %s\n", PyString_AsString(output));
}
int main(int argc, char** argv){
Py_Initialize();
PythonPrinting("print 123");
PythonPrinting("1+5");
PythonPrinting("result = 2");
PythonPrinting("print result");
Py_Finalize();
return 0;
}
The output i get after running main:
Here's the output: 123
Here's the output:
Here's the output:
Here's the output: 2
It is good for me , but only one problem, it should be
Here's the output: 123
Here's the output: 6
Here's the output:
Here's the output: 2
I dont know why but after running this command: PythonPrinting("1+5"), PyString_AsString(output) command returns an empty string (char*) instead of 6... :( Is there somthing i can do not to loose this output?
Thaks, Alex
Answer
Here is a C++ friendly solution I have developed lately.
I explain a few details of it on my blog: Python sys.stdout redirection in C++ where I also point to repository at my GitHub where most recent version can be found. Here is complete example based on the current code at the time of posting this answer:
#include <functional>
#include <iostream>
#include <string>
#include <Python.h>
namespace emb
{
typedef std::function<void(std::string)> stdout_write_type;
struct Stdout
{
PyObject_HEAD
stdout_write_type write;
};
PyObject* Stdout_write(PyObject* self, PyObject* args)
{
std::size_t written(0);
Stdout* selfimpl = reinterpret_cast<Stdout*>(self);
if (selfimpl->write)
{
char* data;
if (!PyArg_ParseTuple(args, "s", &data))
return 0;
std::string str(data);
selfimpl->write(str);
written = str.size();
}
return PyLong_FromSize_t(written);
}
PyObject* Stdout_flush(PyObject* self, PyObject* args)
{
// no-op
return Py_BuildValue("");
}
PyMethodDef Stdout_methods[] =
{
{"write", Stdout_write, METH_VARARGS, "sys.stdout.write"},
{"flush", Stdout_flush, METH_VARARGS, "sys.stdout.flush"},
{0, 0, 0, 0} // sentinel
};
PyTypeObject StdoutType =
{
PyVarObject_HEAD_INIT(0, 0)
"emb.StdoutType", /* tp_name */
sizeof(Stdout), /* tp_basicsize */
0, /* tp_itemsize */
0, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_reserved */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT, /* tp_flags */
"emb.Stdout objects", /* tp_doc */
0, /* tp_traverse */
0, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
0, /* tp_iter */
0, /* tp_iternext */
Stdout_methods, /* tp_methods */
0, /* tp_members */
0, /* tp_getset */
0, /* tp_base */
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
0, /* tp_dictoffset */
0, /* tp_init */
0, /* tp_alloc */
0, /* tp_new */
};
PyModuleDef embmodule =
{
PyModuleDef_HEAD_INIT,
"emb", 0, -1, 0,
};
// Internal state
PyObject* g_stdout;
PyObject* g_stdout_saved;
PyMODINIT_FUNC PyInit_emb(void)
{
g_stdout = 0;
g_stdout_saved = 0;
StdoutType.tp_new = PyType_GenericNew;
if (PyType_Ready(&StdoutType) < 0)
return 0;
PyObject* m = PyModule_Create(&embmodule);
if (m)
{
Py_INCREF(&StdoutType);
PyModule_AddObject(m, "Stdout", reinterpret_cast<PyObject*>(&StdoutType));
}
return m;
}
void set_stdout(stdout_write_type write)
{
if (!g_stdout)
{
g_stdout_saved = PySys_GetObject("stdout"); // borrowed
g_stdout = StdoutType.tp_new(&StdoutType, 0, 0);
}
Stdout* impl = reinterpret_cast<Stdout*>(g_stdout);
impl->write = write;
PySys_SetObject("stdout", g_stdout);
}
void reset_stdout()
{
if (g_stdout_saved)
PySys_SetObject("stdout", g_stdout_saved);
Py_XDECREF(g_stdout);
g_stdout = 0;
}
} // namespace emb
int main()
{
PyImport_AppendInittab("emb", emb::PyInit_emb);
Py_Initialize();
PyImport_ImportModule("emb");
PyRun_SimpleString("print(\'hello to console\')");
// here comes the ***magic***
std::string buffer;
{
// switch sys.stdout to custom handler
emb::stdout_write_type write = [&buffer] (std::string s) { buffer += s; };
emb::set_stdout(write);
PyRun_SimpleString("print(\'hello to buffer\')");
PyRun_SimpleString("print(3.14)");
PyRun_SimpleString("print(\'still talking to buffer\')");
emb::reset_stdout();
}
PyRun_SimpleString("print(\'hello to console again\')");
Py_Finalize();
// output what was written to buffer object
std::clog << buffer << std::endl;
}
This allows to intercept sys.stdout.write output with any kind of callable C++ entity: free function, class member function, named function objects or even anonymous functions as in the example above where I use C++11 lambda.
Note, this is a minimal example to present the essential concept. In production-ready code, it certainly needs more attention around reference counting of PyObject, getting rid of global state, and so on.