Python C-API: How to use the returned python dictionary in Objective-C?

Question

My Python script app.py has this function for example:

def testmethod(x):
    return {"steps":["hello","world"]}

I am using Python C-API in Objective-C as follows:

Py_Initialize();
PyObject *pName = PyUnicode_DecodeFSDefault("app");
PyObject* pModule = PyImport_Import(pName);
Py_DECREF(pName);
        
PyObject* myFunction = PyObject_GetAttrString(pModule,(char*)"testmethod");
PyObject* args = PyTuple_Pack(1,PyFloat_FromDouble(223.22));
PyObject* myResult = PyObject_CallObject(myFunction, args);
NSLog(@"Check for dict: %d",PyDict_Check(myResult)); // Prints true!!!!!!!!!!!! So definitely a dictionary

After this, I am not really sure how I can convert the myResult to retrieve my dictionary. For doubles and strings, we usually use the PyFloat_AsDouble and PyUnicode_AsUTF8String, PyBytes_AsString functions but I can't seem to find a PyBLAHBLAH_AsDict or equivalent method.

double result = PyFloat_AsDouble(myResult);
NSLog(@"Result: %f",result);*/

PyObject* pStrObj = PyUnicode_AsUTF8String(myResult);
char* zStr = PyBytes_AsString(pStrObj);
NSLog(@"String: %@",[NSString stringWithUTF8String:zStr]);

I don't have much experience with Objective-C so maybe I am not looking at the right place.

How can I convert the PyObject to a NSDictionary?

I am basically looking for a C way of accessing the dictionary first (a map for example) which I can then use Objective-C methods to convert to NSDictionary. Similar to how I used PyUnicode_AsUTF8String, PyBytes_AsString and stringWithUTF8String all combined together to get a NSString out of the Python String. Looking for something similar for dictionaries.

I have figured out a way to achieve this but I don't know if this is the best possible way of doing this. I modified my python script to use json.dumps to return a string representation of the dictionary. Then in my Objective-C code, I first get the string, then use JSONObjectWithData to convert the json string to a NSDictionary. This works for now but I am curious if there is a better way of doing this without having to return string instead of dictionary from Python. This seems more like a workaround solution to me. Here's what I have now:

app.py:

import json
def testmethod(x):
    return json.dumps({"steps":["hello","world"]})

Objective-C:

Py_Initialize();
PyObject *pName = PyUnicode_DecodeFSDefault("app");
PyObject* pModule = PyImport_Import(pName);
Py_DECREF(pName);
        
PyObject* myFunction = PyObject_GetAttrString(pModule,(char*)"testmethod");
PyObject* args = PyTuple_Pack(1,PyFloat_FromDouble(223.22));
PyObject* myResult = PyObject_CallObject(myFunction, args);

PyObject* pStrObj = PyUnicode_AsUTF8String(myResult);
char* zStr = PyBytes_AsString(pStrObj);
NSString *jsonStr = [NSString stringWithUTF8String:zStr];
NSLog(@"String: %@",jsonStr);

NSError *error;
NSData *data = [jsonStr dataUsingEncoding:NSUTF8StringEncoding];
NSDictionary *dictionary = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];

NSLog(@"Dictionary: %@",dictionary);

Objective-C is not C or C++. The Python C API is a C API, not an Objective-C API; it doesn't come with functions to convert Python objects into Objective-C data structures. — user2357112
– user2357112, Commented Jul 2, 2018 at 20:03
@user2357112 Yes I understand that. I am basically looking for a C way of accessing the dictionary first (a map for example) which I can then use objective C methods to convert to NSDictionary. Similar to how I used PyUnicode_AsUTF8String, PyBytes_AsString and stringWithUTF8String all combined together to get a NSString out of the Python String. Looking for something similar for dictionaries. — sudoExclamationExclamation
– sudoExclamationExclamation, Commented Jul 2, 2018 at 20:05
Not a direct answer, but have you looked at PyObjC ? pythonhosted.org/pyobjc — koen
– koen, Commented Jul 2, 2018 at 20:07

Artyer · Accepted Answer · 2018-07-02 20:41:35Z

Use PyDict_Next to iterate over key / value pairs of the dictionary.

You should also provide the Python hash and equality functions if you are still going to be using PyObject*s. Otherwise, you should try and coerce them all into native types.

(This is in pseudo code since I don't really know Objective-C, but have used the C-Api and Python)

NSObject* convert_py_object(PyObject* obj) {
    if (PyDict_Check(obj)) return convert_py_dict(obj);
    if (PyLong_Check(obj)) return convert_py_int(obj);
    if (PyUnicode_Check(obj)) return convert_py_str(obj);
    if (PyBytes_Check(obj)) return convert_py_bytes(obj);
    if (PyList_Check(obj) || PyTuple_Check(obj) || PyIter_Check(obj)) return convert_py_list(obj);
    throw error;
}

NSDictionary* convert_py_dict(PyObject* dict) {
    if (!PyDict_Check(dict)) throw error;
    NSDictionary* native_dict;
    PyObject *key;
    PyObject *value;
    Py_ssize_t pos = 0;

    while (PyDict_Next(self->dict, &pos, &key, &value)) {
        native_dict[convert_py_object(key)] = convert_py_object(value);
    }
    return native_dict;
}

Another alternative would be to just use the Python API to access values of a dictionary. So iterate over all the values with PyDict_Next, PyDict_GetItemString to access string items with simple char *s, and PyDict_GetItem to access an arbitrary Python value.

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