Importing module from string in the Python C API

Question

Importing Python modules from files is relatively easy with the Python C API with PyImport_Import() however I would need to use functions stored in strings. Is there a way to import python modules from strings (to clarify: there is no file; the code is in a string) or will I have to save the strings as temp files?

score 4 · Answer 1 · answered Mar 17 '17 at 09:17

No need to use temp files. Use this code:

const char *MyModuleName = "blah";
const char *MyModuleCode = "print 'Hello world!'";
PyObject *pyModule = PyModule_New(MyModuleName);
// Set properties on the new module object
PyModule_AddStringConstant(pyModule, "__file__", "");
PyObject *localDict = PyModule_GetDict(pyModule);   // Returns a borrowed reference: no need to Py_DECREF() it once we are done
PyObject *builtins = PyEval_GetBuiltins();  // Returns a borrowed reference: no need to Py_DECREF() it once we are done
PyDict_SetItemString(localDict, "__builtins__", builtins);

// Define code in the newly created module
PyObject *pyValue = PyRun_String(MyModuleCode, Py_file_input, localDict, localDict);
if (pyValue == NULL) {
    // Handle error
}
else
    Py_DECREF(pyValue);

This is code taken from a real commercial application (I've slightly modified it by removing error handling and other non-needed details). Just set the wanted module name in MyModuleName and the Python code in MyModuleCode and you are done!

Dimitris Fasarakis Hilliard · Answer 2 · 2016-10-01T21:38:14.940

1

~~If my understanding is correct, you could use PyImport_ImportModule which takes a const char* name to specify the module to be imported.~~

Since my understanding was incorrect:

It would generally be better to dump the contents to a .py file and then execute them with PyRun_File but, if you have strings and want to work with those I guess you could use Py_CompileString to compile it to a code object and then feed it to PyEval_EvalCode for evaluation.

edited Oct 01 '16 at 21:38

answered Oct 01 '16 at 15:47

Dimitris Fasarakis Hilliard

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There is no file; the module contents are stored in a string. – Hullu2000 Oct 01 '16 at 15:49
Hm, you can't dump the contents to a file and then import it? I'm pretty sure there's functions to exec valid python code, could you add some examples? – Dimitris Fasarakis Hilliard Oct 01 '16 at 15:52
My application contains a text editing area where the user can type python code and execute it. To execute the code the program puts in function and calls it. – Hullu2000 Oct 01 '16 at 15:54
Dumping it in a temporary file and then letting python parse, compile and eval it should be way easier. If you need to do manually, the functions I added should help you out. – Dimitris Fasarakis Hilliard Oct 01 '16 at 16:10
Is it possible to replace a function with a nother function with the same name and parameters so the code can be edited? – Hullu2000 Oct 01 '16 at 16:14
I seriously doubt that. – Dimitris Fasarakis Hilliard Oct 01 '16 at 16:19

score 0 · Answer 3 · answered Nov 02 '20 at 09:44

I've had success following the strategy laid out by Dimitirs.

            Py_Initialize();
            PyObject *module = Py_CompileString(
                // language: Python
                R"EOT(
fake_policy = {
      "maxConnections": 4,
      "policyDir": "/tmp",
      "enableVhostPolicy": True,
      "enableVhostNamePatterns": False,
})EOT",
                "test_module", Py_file_input);
            REQUIRE(module != nullptr);

            PyObject *pModuleObj = PyImport_ExecCodeModule("test_module", module);
            REQUIRE(pModuleObj != nullptr);
            // better to check with an if, use PyErr_Print() or such to read the error

            PyObject *pAttrObj = PyObject_GetAttrString(pModuleObj, "fake_policy");
            REQUIRE(pAttrObj != nullptr);

            auto *entity = reinterpret_cast<qd_entity_t *>(pAttrObj);
            REQUIRE(qd_entity_has(entity, "enableVhostNamePatterns"));  // sanity check for the test input

            // call Py_DecRef s

I used https://awasu.com/weblog/embedding-python/calling-python-code-from-your-program/ as another reference, when writing this.

Importing module from string in the Python C API

3 Answers3

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