So far, we have seen two different function declarations in Cython, def and cdef, to define functions. There is one more declaration—cpdef. The def is a Python-only function, so it is only callable from Python or Cython code blocks; calling from C does not work. The cdef is the opposite; this means that it's callable from C and not from Python. For example, if we create a function such as:
cpdef public test (int x): … return 1
It will generate the following function prototype:
__PYX_EXTERN_C DL_IMPORT(PyObject) *test(int, int __pyx_skip_dispatch);
The public keyword will make sure we generate the needed header so that we can call it from C. Calling from pure Python, we can work with this as if it was just any other Python function. The drawback of using cpdef is that the native return type is PyObject *, which requires you to know exactly what the return type is and consult the Python API documentation to access the data. I prefer keeping bindings between the languages simpler...
