How do I expose a C variable to Python as a variable?

Question

Suppose in a custom python module written in C, I declare the variable:

static int module_state;

The functions in my module set the state of this variable. If I want to know the value of module_state, I can declare a getter function like this:

PyObject * get_module_state(PyObject *self)
{
   return PyLong_FromLong(module_state);
}

On python (3) I could simply do:

import module
...
state = module.get_module_state()

However, I want to expose it as a variable:

import module
state = module.module_state

I looked at PyMemberDef and PyGetterSetterDef in the API but I am confused about how to use them to expose the variable. Generally speaking I don't want the python user to be able to modify the variable.

The purpose of this variable is very similar to errno, and I want to make it available as transparently as possible.

Does this answer your question? [Exporting C global variables to a python extension](https://stackoverflow.com/questions/17793353/exporting-c-global-variables-to-a-python-extension) — mkrieger1, Jul 27 '20 at 10:35
The suggested question seems to deal with constants, which I am already familiar with. However module_state can change during program runtime and I want to make each change available. — gny-001f2, Jul 27 '20 at 10:42
This is relevant (although pure Python) https://stackoverflow.com/questions/880530/can-modules-have-properties-the-same-way-that-objects-can. — DavidW, Jul 27 '20 at 11:39
@DavidW yes I could use properties to work around it, but that adds two layers - an internal C ```_module``` and a user facing python ```module```. — gny-001f2, Jul 27 '20 at 11:45
Or a `__getattr__` function (if you're using a suitable version of Python?). I dunno if there's a better way of doing it — DavidW, Jul 27 '20 at 13:02
I believe that has the same problem as using properties. Since __getattr__ has to be declared in your python code while my module is written entirely in C. — gny-001f2, Jul 27 '20 at 14:36
I haven't tried it, but I don't see why you couldn't define a C function called `__getattr__`. I'll give it a go... — DavidW, Jul 27 '20 at 16:15

score 0 · Accepted Answer · answered Jul 27 '20 at 16:40

The two main suggestions in Can modules have properties the same way that objects can? can actually be translated pretty "easily" to C.

A module-level __getattr__. This needs Python 3.7. You just define a function called __getattr__ using the normal C API mechanism, PyMethodDef.

This function would do something like:
```
if (PyUnicode_CompareWithASCIIString(arg, "name") == 0) {
    return PyLong_FromLong(your_variable);
}
else {
    PyErr_SetObject(PyExc_AttributeError, arg);
    return NULL;
}
```
Note that the limitation (which I think you're OK with) is that variable would be read-only since there isn't an equivalent module __setattr__.
Use a class instead of the module. This can be a class defined in C, with PyMemberDef exposing the C variable as a property.

This probably works best for a submodule (e.g. main_mod.submod). That way the main module could add your class instance to sys.modules something like:
```
PyObject *sys_modules = PySys_GetObject("modules");
PyDict_SetItemString(sys_modules, "main_mod.submod", instance);
```
I think this way's probably much more trouble than the first option, but it would work in C.

How do I expose a C variable to Python as a variable?

1 Answers1