Python C API - Reload a module

Question

I use Python 3.4 and Visual 2010. I'm embedding Python using the C API to give the user some script capabilities in processing his data. I call python functions defined by the user from my C++ code. I call specific function like Apply() for example that the user has to define in a Python file. Suppose the user has a file test.py where he has defined a function Apply() that process some data. All I have to do is to import his module and get a "pointer" to his python function from the C++.

PySys_SetPath(file_info.absolutePath().toUtf8().data()));
m_module = PyImport_ImportModule(module_name.toUtf8().data());
if (m_module)
{
    m_apply_function = PyObject_GetAttrString(m_module, "Apply");
    m_main_dict = PyModule_GetDict(m_module);
}

So far, so good. But if the user modifies his script, the new version of his function is never taken into account. I have to reboot my program to make it work... I read somewhere that I need to reload the module and get new pointers on functions but the PyImport_ReloadModule returns NULL with "Import error".

// .... code ....
// Reload the module
m_module = PyImport_ReloadModule(m_module);

Any ideas ? Best regards, Poukill

The module most probably isn't loading. Can you add a PyErr_Print to see if there's an exception and which exception is it ? — Tasos Vogiatzoglou
– Tasos Vogiatzoglou, Commented Feb 13, 2015 at 12:20
I can't use PyErrorPrint since I'm using the Python C API. But with some inspection, I have a "Import can't find module, or can't find name in module." Which is weird because I just loaded the module at this very same place (PySys_SetPath has been used for that). If I use PyImport_ImportModule instead, it's working, I get a valid pointer, but not updated. — poukill
– poukill, Commented Feb 13, 2015 at 16:25
PyErr_Print is a c api function, an alias to PyErr_PrintEx. There are a few caveats around reloading and name resolution. Is there a possibility to just restart the interpreter ? — Tasos Vogiatzoglou
– Tasos Vogiatzoglou, Commented Feb 13, 2015 at 17:22
have you tried importlib.reload(module)? Calling PyImport_ImportModule the second time is pointless; it probably just returns the cached value from sys.modules. Make sure PySys_SetPath argument contains all necessary paths (including pythonpath that is necessary for the user script). — jfs
– jfs, Commented Feb 14, 2015 at 12:36
@TasosVogiatzoglou : I know PyErr_Print is a C function, but it's printing directly into the python interpreter (sys.stderr) which I can not see. Like I said, I embed Python so I don't see the outputs of the interpreter for now, I have to capture the exception and the stacktrace using PyErr_Fetch(&ptype, &pvalue, &ptraceback), but it's not that easy. Restarting the interpreter may be a option, but clearly not the best as I would have to stop any running session. — poukill
– poukill, Commented Feb 16, 2015 at 8:30

poukill · Accepted Answer · 2015-02-16 10:28:25Z

1

The answer was found in the comments of my first post (thank you J.F Sebastian), the PySys_SetPath has to contain also the PYTHONPATH. In my case, that is the reason why the PyImport_ReloadModule was failing.

QString sys_path = file_info.absolutePath() + ";" + "C:\\Python34\\Lib";
PySys_SetPath(UTF8ToWide(sys_path.toUtf8().data()));
m_module = PyImport_ReloadModule(m_module); // Ok !

answered Feb 16, 2015 at 10:28

poukill

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