How to Pass Two-dimensional array from C to Python

Question

I want to pass the two-dimensional array to python from C.

How can I use the Py_BuildValue() and PyEval_CallObject()?

For example, i can use the following code to pass string from C to python:

pModule = PyImport_ImportModule("python_code");
pFunc = PyObject_GetAttrString(pModule, "main");
pParam = Py_BuildValue("(s)", "HEHEHE");
pResult = PyEval_CallObject(pFunc,pParam);

Now, i want to pass the two-dimensional array and a string to python

Do you with to return a list or to alter a list argument? If you don't know then returning is easier. — cdarke
– cdarke, Commented May 19, 2015 at 15:48
You might also wish to consider ctypes in the standard library. — cdarke
– cdarke, Commented May 19, 2015 at 16:08
can you show us your code and sample data you want to pass to python? — m.wasowski
– m.wasowski, Commented May 19, 2015 at 16:49
I just want to pass the "vector<vector<int> > arr" to python function, in python, it can be tuple or list — vinllen
– vinllen, Commented May 20, 2015 at 2:41

mauriii · Accepted Answer · 2019-07-08 19:09:04Z

4

So basically, you want to build a tuple, not parse one.

This is just a straightforward example of how you could convert your arr to a tuple of tuples. Here you should add some error checking at some point in time as well.

Py_ssize_t len = arr.size();
PyObject *result = PyTuple_New(len);
for (Py_ssize_t i = 0; i < len; i++) {
    Py_ssize_t len = arr[i].size();
    PyObject *item = PyTuple_New(len);
    for (Py_ssize_t j = 0; j < len; j++)
        PyTuple_SET_ITEM(item, j, PyInt_FromLong(arr[i][j]));
    PyTuple_SET_ITEM(result, i, item);
}

(For Python 3 C API, replace PyInt_FromLong(arr[i][j]) with PyLong_FromLong(arr[i][j]))

Then you can build your args, like you did with the string. Instead of s for string, you would use O for PyObject * (or N if you don't want to increment the reference count):

pParam = Py_BuildValue("(O)", result);

Maybe boost::python could provide a simpler method, but I don't realy know the library myself.

edited Jul 8, 2019 at 19:09

mauriii

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answered May 20, 2015 at 10:08

tynn

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1 Comment

vinllen Over a year ago

Thankyou tynn, then how to send the value to python scripty after building the tuple. For example, i used this code to send the string: pParam = Py_BuildValue("(s)", "HEHEHE"); pResult = PyEval_CallObject(pFunc,pParam);

user2108206 · Accepted Answer · 2018-01-09 13:54:34Z

0

second loop needs a subtile change from:

for (Py_ssize_t j = 0; i < len; j++)

to

for (Py_ssize_t j = 0; j < len; j++)

with the right running condition statement.

answered Jan 9, 2018 at 13:54

user2108206

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