Getting Python function argument names in C++

Question

I'm trying to get the argument names of a Python function called from C++. A little background:

I have a C++ application that needs to call Python functions, passing them arguments by name. So far I have been able to do this by parsing the Python module's .py file; however I would like to be able to handle .pyc files as well.

Ideally this would be done via the Python C API (rather than trying to decompile the bytecode), but I can't see any obvious way of doing this. There are ways of doing this in Python e.g. with inspect but not in C++.

Anyone know of a possible solution? TIA.

Answer 1

You said that you would use the inspect module if you wanted to do this from python. Why not call the inspect module from the C API?

The following C code prints all arguments of a python function. It should be valid C++ code as well. I dont use the Python C API very often, so please tell me if some things could be improved.

#include <Python.h>
#include <stdio.h>

int main(int argc, char* argv[])
{
    wchar_t* const program = Py_DecodeLocale(argv[0], nullptr);
    Py_SetProgramName(program);
    Py_Initialize();

    // Define a python function f with argument names x and y.
    // >>> def f(x, y): return x*y
    PyRun_SimpleString("def f(x, y): return x*y\n");

    // Get the function as python object.
    // >>> import sys
    // >>> f_function = sys.modules["__main__"].f
    PyObject* const sys_module_name = PyUnicode_DecodeFSDefault("sys");
    PyObject* const sys_module = PyImport_Import(sys_module_name);
    PyObject* const modules_dict = PyObject_GetAttrString(sys_module, "modules");
    PyObject* const main_name = PyUnicode_DecodeFSDefault("__main__");
    PyObject* const main_module = PyDict_GetItem(modules_dict, main_name);
    PyObject* const f_function = PyObject_GetAttrString(main_module, "f");

    // Get the inspect.getargspec function.
    // >>> import inspect
    // >>> getargspec_function = inspect.getargspec
    PyObject* const inspect_module_name = PyUnicode_DecodeFSDefault("inspect");
    PyObject* const inspect_module = PyImport_Import(inspect_module_name);
    PyObject* const getargspec_function = PyObject_GetAttrString(inspect_module, "getargspec");

    // Call the inspect.getargspec function.
    // >>> argspec = getargspec_function(f_function)
    PyObject* const argspec_call_args = PyTuple_New(1);
    PyTuple_SetItem(argspec_call_args, 0, f_function);
    PyObject* const argspec = PyObject_CallObject(getargspec_function, argspec_call_args);

    // Get args from argspec.
    // >>> f_args = argspec.args
    PyObject* const f_args = PyObject_GetAttrString(argspec, "args");

    // f_args now holds a python list with all arguments of f.

    // As example usage, you can print the arguments:
    // >>> for i, a in enumerate(f_args):
    // ...     print("Repr of arg", i, "is", repr(a))
    Py_ssize_t const num_args = PyList_Size(f_args);
    for (Py_ssize_t i = 0; i < num_args; ++i)
    {
        PyObject* const arg = PyList_GetItem(f_args, i);
        PyObject* const arg_repr = PyObject_Repr(arg);
        PyObject* const arg_str = PyUnicode_AsASCIIString(arg_repr);
        char const* const arg_c_str = PyBytes_AS_STRING(arg_str);
        printf("Repr of arg %ld is %s\n", i, arg_c_str);
    }

    Py_Finalize();
    PyMem_RawFree(program);

    return 0;
}