Cython-generated unbound class methods cannot be pickled

[pierreglaser]

Building the following module (src/module.py)

class MyClass:

    def __init__(self, a):
        self.a = a

    def my_method(self, b):
        return self.a + b

Using cython with the following setup script:

from distutils.core import setup
from Cython.Build import cythonize

setup(ext_modules=cythonize("src/module.py"))

Generates a class (MyClass), whose unbound methods cannot be pickled:

In [1]: from module import MyClass                                                                                                                                                                                 
                                                                                                                                                                                                                   
In [2]: import pickle                                                                                                                                                                                              
                                                                                                                                                                                                                   
In [3]: pickle.dumps(MyClass.my_method)                                                                                                                                                              
---------------------------------------------------------------------------                                                                                                                                        
PicklingError                             Traceback (most recent call last)                                                                                                                                        
<ipython-input-3-cf09bde7c7ba> in <module>                                                                                                                                                                         
----> 1 pickle.dumps(MyClass.my_method)                                                                                                                                                              
                                                                                                                                                                                                                   
PicklingError: Can't pickle <cyfunction MyClass.my_method at 0x7f8d130e9ea8>: attribute lookup
my_method on module failed

[pierreglaser]

Note that this bug only affects non-extension cython classes:

• if MyClass is defined as an extension type (using cdef) then MyClass.my_method becomes a method_descriptor, which is picklable by default.
• otherwise, MyClass.my_method is a cyfunction, with an invalid __reduce__ method.