Question: pickling skeletons

[jrjbertram]

Hi there,

I tried pickling a polygon-with-holes straight skeleton today and got an error. Example code to illustrate what I'm doing:

poly = sg.PolygonWithHoles(shell, holes )
skel = sg.skeleton.create_interior_straight_skeleton(poly)
skeleton = { 'poly': poly, 'skel': skel }
pickle.dump( skeleton, open( 'skeleton.pkl', 'wb') )

error:

TypeError: cannot pickle 'skgeom._skgeom.PolygonWithHoles' object

My hope was to cache off this data structure as it takes quite a while to compute for my problem. Is this a bug, a limitation, or just something I'm doing wrong? Or do I need to unpack everything I need out of the skel object into lists/dictionaries and pickle that instead?

Didn't see any similar issues for this project and don't see a forum identified for questions, so adding this issue here. Apologies if there is a preferred other place to ask (e.g., stack overflow, etc.)

Thank you!
Josh.

[jrjbertram]

Doing some more digging on this question. It's looking like the answer is no from here:
https://docs.python.org/3/library/pickle.html#what-can-be-pickled-and-unpickled

Specifically I think this rule is probably being violated:

• classes that are defined at the top level of a module

For example here where classes are being defined within the C++ implementation, though I'm not 100% on whether this technically violates the rule.

https://github.com/scikit-geometry/scikit-geometry/blob/master/src/skeleton.cpp#L84

And there is no fallback provided per this rule:

• instances of such classes whose dict or the result of calling getstate() is picklable

So if I'm understanding everything correctly, to support pickling, each class would need to have a getstate member function added to it which returned a dictionary that captured its state, along with a setstate member function would would allow this state to be restored (with appropriate supporting CGAL calls assuming they are available.) Something along the lines of:

https://docs.python.org/3/library/pickle.html#handling-stateful-objects