Python debug shell in embedded environment: exec or eval? - c++

Question

I'm building an embedded python application in C++, using boost::python. The embedded environment exports part of itself as a module, in other words the python code that runs in the environment won't run in a standard python env as the embedded module can't be imported.
One of the features that would be really helpful for debugging would be a debug shell, where I can break out and manually input standard python expressions to inspect/modify the current state. The problem I have is that I don't/can't know in advance whether it's an eval (e.g. "2+2") or an exec (e.g. "a=2+2"). If I exec("2+2") I don't get the result, and if I eval("a=2+2") I get a syntax error. (Coming from a C background I don't quite understand why this distinction exists). Trying eval first, then exec if it fails, seems like a very dubious solution not least because of side effects.
As far as I can see boost::python simply replicates python's eval/exec functions, so this problem is more at the python level, but the answer I'm hoping to get is how to do it at the C++ (ideally boost) level. Is there some horrendously complicated regex that could be used to differentiate exec-able from eval-able code? For now I'm just going with the ugly solution of making the user decide with a prefix on the expression.
Here's an MCVE with a TODO at the point where I need to choose between eval and exec (obv. requires python-dev and boost_python libs)
// g++ $(python3-config --cflags) so.cpp -fPIC -lboost_python3 $(python3-config --ldflags)
#include <boost/python.hpp>
#include <string>
#include <iostream>
namespace py = boost::python;
std::string pyobject_to_string(PyObject* obj)
{
PyObject* repr = PyObject_Str(obj);
PyObject* str = PyUnicode_AsEncodedString(repr, "utf-8", "~E~");
const char *bytes = PyBytes_AS_STRING(str);
Py_XDECREF(repr);
Py_XDECREF(str);
return std::string(bytes);
}
int main(int argc, const char** argv)
{
// Init python env
Py_Initialize();
if (argc < 2)
{
return 1;
}
try
{
// TODO decide, based on argv[1], which needs to be called, this:
py::object res = py::eval(argv[1]);
std::cout << pyobject_to_string(res.ptr()) << std::endl;
// or this:
py::exec(argv[1]);
// or something else entirely...?
}
catch(py::error_already_set&)
{
if (PyErr_Occurred())
{
PyObject *type, *value, *traceback;
PyErr_Fetch(&type, &value, &traceback);
std::string message = pyobject_to_string(type) + ":" + pyobject_to_string(value);
//PyErr_Restore(type, value, traceback);
std::cerr << message << std::endl;
}
else
{
std::cerr << "unknown error" << std::endl;
return 1;
}
}
}
Example output is:
$ ./a.out 2+2
4
$ ./a.out a=2+2
<class 'SyntaxError'>:('invalid syntax', ('<string>', 1, 2, 'a=2+2'))
Many thanks in advance