Cython caveats

There are some caveats worth noting while mixing C and the Python code when it comes to Cython. It's a good idea to refer to these when building something to be production ready.

Type checking

You may have noticed that in the previous code examples, we were able to cast the void * pointer from malloc to our extension types using malloc. Cython supports some more advanced type checking as follows:

char * buf = <char *> malloc (sizeof (...))

In basic type casting, Cython supports <type?> for type checking:

char * buf  = <char *?> malloc (...)

This will do some type checking and throw an error if the type that is being cast is not a subclass of char *. So, in this case, it will pass; however, if you were to do the following:

cdef class A:
     pass
cdef class B (A):
     pass

def myfunc ():
    cdef A class1 = A ()
    cdef B class2 = B ()
    cdef B x = <B?> class1

This will return an error (at runtime):

Traceback (most recent call last):
  File "main.py", line...