Question

I have a function wrapped in Cython:

cdef extern  from "myheader.h":
   int c_my_func (const_char *a, const_char* b)

And a function exposed to Python:

def my_func(a, b):
    c_my_func(a, b)

The function c_my_func accepts NULL for parameters a and b. When I call it from the interpreter:

my_func(None, None)

it throws the exception:

TypeError: expected string or Unicode object, NoneType found

How do I make this function accept the None and pass NULL to the c_my_func ? I don’t want to manually check for the None and then pass NULL. I also tried using default parameters on the cdef of the c_my_func and it doesn’t worked.

Answer 1

You can find the answer in the Cython FAQ.

None is not compatible with any C type. To accommodate for this, the
default behavior is for functions with cdefed parameters to also
accept None.

1. If you want to consider None invalid input, then you need to write
   code that checks for it, and raised an appropriate exception.

The solution is to manually check for None and pass NULL, as you write.