Lately, I have been embedding Python interpreter and implementing plenty of Python extensions in C++ using plain C API provided by Python 3. One of common challenges at C/C++ level is to intercept output sent to sys.stdout or sys.stderr by Python functions like print. Python Embedding/Extending FAQ suggests common solution based on Python code:
# catcher code
import sys
class StdoutCatcher:
def __init__(self):
self.data = ''
def write(self, stuff):
self.data = self.data + stuff
catcher = StdoutCatcher()
sys.stdout = catcher
This Python code can be executed by embedded Python interpreter using PyRun_SimpleString, then the output can be accessed by fetching __main__ module attributes:
PyObject* m = PyImport_AddModule("__main__");
char const* code = "... catcher code here...";
PyRun_SimpleString(code);
PyRun_SimpleString("print(3.14)");
PyObject* catcher = PyObject_GetAttrString(m, "catcher");
PyObject* output = PyObject_GetAttrString(catcher, "data");
// get textual data contained in output
Such mix of Python and C code is neither convenient to use nor states a flexible solution. I simply don’t like this prosthesis, especially if I need to frequently switch between number output sinks.
So, I have come up with better solution which allows me to directly bind any callable C++ entity. The syntax I mean looks and feels like this:
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 me to handle sys.stdout.write with C++ free function, class member function, named function objects or even anonymous functions as in the example above where I use C++11 lambda.
Complete implementation of the emb module in C/C++ using plain Python C API is available from my Python workshop at GitHub:
git clone git://github.com/mloskot/workshop.git
The complete code is enclosed in python/emb/emb.cpp file. 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 one.
