Create Python Constructor with parameter from C++, PyObject

Question

I have a python class A like this.

class A: 
   def __init__(self, name): 
       self.name = name

   def print_lastname(self, lastname): 
       print(lastname)

I have to call this code like this.

import B
a = B.A("hello")
a.print_lastname("John")

Currently, I need to use this A class from my C++ code. I have got this far.

Py_Initialize(); 
string hello = "hello"; 
PyObject *module, *attr, *arg; 
module = PyObject_ImportModule("B"); // import B
attr = PyObject_GetAttrString(module, "A"); // get A from B
arg = PyString_FromString(hello.c_str()); 
instance = PyInstance_New(attr, arg, NULL); // trying to get instance of A with parameter "hello"
Py_Finalize();

But I am getting error

Exception TypeError: 'argument list must be tuple' in module 'threading' from '/usr/lib64/python2.7/threading.pyc'

How can I achieve from import statement to a.print_name("John") from C++? Any help is appreciated.

John Q. · Accepted Answer · 2016-10-20 04:04:27Z

6

I'm going to rewrite the Python class slightly, just so it uses both the argument and member variable.

# B.py - test module
class A:
    def __init__(self, name):
        self.name = name

    def print_message(self, message):
        print message + ' ' + self.name

As for the C++ part, almost everything looks okay. The error you're getting is because the argument to PyInstance_New should be a tuple. There are multiple ways to call functions or methods. Here is a complete example using one of them:

// test.c - test embedding.
void error_abort(void)
{
    PyErr_Print();
    exit(EXIT_FAILURE);
}

int main(int argc, char* argv[])
{
    PyObject* temp, * args, * attr, * instance;

    Py_Initialize();
    if (!(temp = PyString_FromString("John")))
        error_abort();
    if (!(args = PyTuple_Pack(1, temp)))
        error_abort();
    Py_DECREF(temp);

    if (!(temp = PyImport_ImportModule("B")))
        error_abort();
    if (!(attr = PyObject_GetAttrString(temp, "A")))
        error_abort();
    Py_DECREF(temp);

    if (!(instance = PyInstance_New(attr, args, NULL)))
        error_abort();
    if (!PyObject_CallMethod(instance, "print_message", "s", "Hello"))
        error_abort();

    Py_DECREF(args);
    Py_DECREF(attr);
    Py_DECREF(instance);
    Py_Finalize();
    return 0;
}

For more information see Python pure-embedding.

edited Oct 20, 2016 at 4:04

answered Oct 20, 2016 at 1:59

John Q.

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2 Comments

pseudo Over a year ago

very awesome! Works like a charm.

Alan Over a year ago

Note that PyInstance_New is not available in Python 3.x.

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