Preventing subclassing

If you create an extension type in Cython, something you never want to be subclassed, it is a cpp class wrapped in a Python class. To prevent this, you can do the following:

cimport cython

@cython.final
cdef class A: pass

cdef class B (A): pass

This annotation will give an error when someone tries to subclass:

pycode.pyx:7:5: Base class 'A' of type 'B' is final

Note that these annotations only work on the cdef or cpdef functions and not on normal Python def functions.