Python SystemError when using the C API PyRun_String

Question

I am trying to run, in a C++ program, some python file that is actually included in the C++ code as a std::string called command.

I have been successful using PyRun_SimpleString(command.c_str()), but this prevents getting exception information.

I now try the following instead:

PyObject* result = PyRun_String(command.c_str(), Py_file_input, PyEval_GetGlobals(), PyEval_GetLocals());

However, this throws:

SystemError: frame does not exist

What am I doing wrong?

UPDATE:

I tried to split the process into compile+eval using the following:

PyObject* code = Py_CompileString(command.c_str(), filename.c_str(), Py_file_input);
PyObject* result = PyEval_EvalCode(code, PyEval_GetGlobals(), PyEval_GetLocals());

Now, the error is different:

SystemError: PyEval_EvalCodeEx: NULL globals

This is maybe more explanatory, as it says in the C API doc::

Return a dictionary of the global variables in the current execution frame, or NULL if no frame is currently executing.

I am not sure I understand what a frame is.

@p-a-o-l-o all commands fail. I tried, for instance, "import math" or "print(1)" — fffred
– fffred, Commented Dec 19, 2017 at 13:17

fffred · Accepted Answer · 2017-12-19 17:24:32Z

4

I found a solution. I probably have not understood when a frame is exectuting or not, but it turns out that just after launching the interpreter, no frames are running. That's why PyEval_GetGlobals() and PyEval_GetLocals() were returning NULL and caused errors later.

Instead, the namespace of the default module __main__ should be used as both globals and locals namespaces:

PyObject* d = PyModule_GetDict(PyImport_AddModule("__main__"));
PyObject* result = PyRun_String(command.c_str(), Py_file_input, d, d);
Py_DECREF(result);

answered Dec 19, 2017 at 17:24

fffred

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