[FIXED] How to override property setter in a subclass of a Cython extension type

Issue

I have a Python module which defines class A with subclass B. Class A defines a property with a getter and setter, class B overrides the setter:

import abc

class A:
    __metaclass__ = abc.ABCMeta

    def __init__(self):
        self.__x = 0

    @property
    def x(self):
        return self.__x

    @x.setter
    def x(self, newX):
        print("Setter in class A")
        self.__x = newX

class B(A):
    @A.x.setter
    def x(self, newX):
        print("Setter in class B")
        A.x.fset(self, newX)

b = B()
b.x = 42
print(b.x)

When I let Cython compile the module and then import it, Python prints the desired output:

Setter in class B
Setter in class A
42

Running it as a pure Python module (without Cython compilation) also gives me the desired output.

For performance reasons, I'd like to turn A into an extension type by changing line 3 to:

cdef class A:

With this, Cython compilation remains possible, but then I get this exception at runtime: File "test.pyx", line 21, in init test @A.x.setter AttributeError: 'getset_descriptor' object has no attribute 'setter'

Is there a different way to override the setter, so that it also works with a cdef class?

Solution

For performance reasons, I'd like to turn A into an extension type [...]

One thing worth emphasising - by using a cdef class you are trading flexibility for performance so this sort of thing not working is expected.

What I'd do in this case is have the setter in A call an implementation function, and override the implementation function in B. If B is a cdef class then the implementation function can be cdef. Otherwise it needs to be cpdef (or def):

cdef class A:
    ...

    @x.setter
    def x(self, newX):
        self.xSettingImpl(newX)

    cpdef xSetterImpl(self, newX):
        print("Setter in class A")
        self.__x = newX

class B:
    def xSetterImpl(self, newX):
        print("Setter in class B")
        A.x.fset(self, newX)  # Not sure this is likely to work though...

---

Also, I don't think this will work in a cdef class:

    __metaclass__ = abc.ABCMeta

Answered By - DavidW