How to create and assign a Python variable in C?

Question

I've initialized the Python environment by

Py_Initialize();

I have no external Python module imported into the environment and everything works well. But when I need to pass a C string into this environment, I am lost...

I was thinking to add a function in the environment to assign the variable like the following code do.

char *str;
str="\
def assign_var(str):\
    global string\
    string = str";
PyRun_SimpleString(str);

And then call this function in C and pass the converted C string as the arguments.

I don't think all I mentioned above is a good way solve the problem...

How can I make this work?

Solution:

Finally, here's the solution with Peter Mortensen's help. (Thanks Peter Mortensen!)

As the python environment I've initialized is a pure empty environment(without any imported modules). I use

py_main = PyImport_AddModule("__main__");

to get a hook to the main environment. and then call

PyModule_AddStringConstant(py_main, "string_name", str);

to bring the C string into the python environment.

To verify everything is done, just try:

PyRun_SimpleString("print dir()");
PyRun_SimpleString("print string_name");

and you'll see you "string_name" string appears in the dir() list and make it print by python!

@senderle sorry, that case, he already has a variable g in the module, but in my case I've none... thanks to your comment, I've reprecised the title of my question(added 'creat') — alwinlin
– alwinlin, Commented Jun 8, 2011 at 16:45
my thought was that you might be able to acquire a reference to __main__ using PySys_GetObject. But without knowing exactly what you're doing it's a bit hard to tell what you want to happen. — senderle
– senderle, Commented Jun 9, 2011 at 3:56

JAB · Accepted Answer · 2011-06-08 23:10:52Z

2

This should do what you want:

char *cStr = "Some text here.";

PyObject *pyStr = Py_BuildValue("s", cStr);

http://docs.python.org/c-api/arg.html#Py_BuildValue

Of course if you're using Python 3 (or use it in the future), there may be situations where you'd want to use "y" instead of "s" and get a bytes object rather than a str.

UPDATE: Woops, I forgot the even easier way of doing it.

PyObject *pyStr = PyString_FromString(cStr);

http://docs.python.org/c-api/string.html#PyString_FromString

(It'd be PyBytes_FromString() in Python 3.)

You might want to take a look at http://docs.python.org/extending/embedding.html for some more information.

Here's something else you might want to try. See

http://docs.python.org/c-api/module.html#PyModule_AddObject
Or possibly
http://docs.python.org/c-api/module.html#PyModule_AddStringConstant

With the former it'd be something like

errorcheck = PyModule_AddObject(embmodule, "str", pyStr);

And with the latter, something like

errorcheck = PyModule_AddStringConstant(embmodule, "str", cStr);

edited Jun 8, 2011 at 23:10

answered Jun 8, 2011 at 19:55

JAB

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4 Comments

alwinlin Over a year ago

sorry, this is not what I'm looking for... all the functions you mentioned here return always PyObject* and nothing happens in the python environment. The python string stays still in C environment not in the python one. Do you have any idea about transfering the python string into python environment from the C one?

JAB Over a year ago

Look at docs.python.org/extending/…. You'll need to write a function that returns the string as a PyObject using the same PyString_FromString() function I showed, make an array of PyMethodDef objects, and then initialize a module with that array of methods. You'd then import your module from the Python side and call the methods of the module to access the data.

JAB Over a year ago

PyMethodDef structs, rather.

alwinlin Over a year ago

Hi, sorry for my late response. I've finally found a way to creat and assign a string. As you mentioned, I'm using Module_AddStringConstant(mod, "name", cStr);

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