Cython classes are non-pickable

[smcantab]

Pele Cython classes are currently non pickable, they require a reduce method to be able to recreate a copy of the original object after being unpicked. This is because Pickle does not know anything about the nature of extension types. I came across this problem trying to parallelise some code with multiprocessing.

Official documentation about the reduce method is available here
https://docs.python.org/2/library/pickle.html#pickling-and-unpickling-extension-types
I found it not very informative though due to lack of examples and in general not many examples are available on the web, this is a point lamented even by the cython developers.

This is a minimal example of the approach I have taken so far:

cdef class PickableCythonClass:
    cdef potential
    cdef coords
    cdef __cinit__(pot, x):
        cdef _pele.BasePotential potential = pot
        cdef np.ndarray[dtype='d'] coords = x
        self.potential = potential
        self.coords = x
   cdef __reduce__(self):
          return (PickableCythonClass,(self.potential, self.coords))
          #note that the attributes have been passed as a tuple

if you think there's a better approach, please let me know.