13. Source Code Layout

I find it useful to physically separate out the source code into different categories:

Category	Language	#include <Python.h>?	Testable?	Where?	Description
Pure Python	Python	No	Yes	py/	Regular Python code tested by pytest or similar.
CPython interface	Mostly C	Yes	No	cpy/	C code that defines Python modules and classes. Functions that are exposed directly to Python.
CPython utilities	C, C++	Yes	Yes	cpy/	Utility C/C++ code that works with Python objects but these functions that are not exposed directly to Python. This code can be tested in a C/C++ environment with a specialised test framework. See C++ RAII Wrappers Around PyObject* for some examples.
C/C++ core	C, C++	No	Yes	cpp/	C/C++ code that knows nothing about Python. This code can be tested in a C/C++ environment with a standard C/C++ test framework.

13.1. Testing CPython Utility Code

When making Python C API calls from a C/C++ environment it is important to initialise the Python interpreter. For example, this small program segfaults:

#include <Python.h>

int main(int /* argc */, const char *[] /* argv[] */) {
    /* Forgot this:
    Py_Initialize();
    */
    PyErr_Format(PyExc_TypeError, "Stuff",);
    return 0;
}

The reason is that PyErr_Format calls PyThreadState *thread_state = PyThreadState_Get(); theen thread_state will be NULL unless the Python interpreter is initialised.

So you need to call Py_Initialize() to set up statically allocated interpreter data. Alternativley put if (! Py_IsInitialized()) Py_Initialize(); in every test. See: https://docs.python.org/3/c-api/init.html

Here are a couple of useful C++ functions that assert all is well that can be used at the begining of any function:

/* Returns non zero if Python is initialised and there is no Python error set.
 * The second version also checks that the given pointer is non-NULL
 * Use this thus, it will do nothing if NDEBUG is defined:
 *
 * assert(cpython_asserts());
 * assert(cpython_asserts(p));
 */
int cpython_asserts() {
    return Py_IsInitialized() && PyErr_Occurred() == NULL;
}

int cpython_asserts(PyObject *pobj) {
    return cpython_asserts() && pobj != NULL;
}