Make Python class inherited from boost-python class copyable

Question

I have a class that inherits from a boost-python class:

class Magnet(CMagnet):   # CMagnet is a C++ based boost-python class
    def __init__(self):
        CMagnet.__init__(self)

    def python_method(self):
        ...

In the C++ implementation of CMagnet I used the code from 1, as posted in 2.

I now have the following problem: When I do the following:

magnet = Magnet()
magnet_2 = copy.deepcopy(magnet)

then magnet is of type Magnet, magnet_2, however, is of type CMagnet. I need it to be also of type Magnet. It lacks all Magnet methods. How do I get deepcopy to copy (and return) the entire Magnet object and not only a copy of the CMagnet part?

There's a related question which indicates that you should implement the __deepcopy__ yourself, but I don't know if you're already doing it, or if your implementation have any bug; like @Leon said, it's better if you provide a reproducible example. — Terseus
– Terseus, Commented Jun 4, 2019 at 16:16

Leon · Accepted Answer · 2019-06-06 07:51:49Z

4

+50

Since you didn't provide a Minimal, Reproducible Example I cannot quickly check if the following dirty trick works, but I think that it should.

You can add a __deepcopy__() method to your class that delegates the work to the underlying boost-python object and then fixes the type of the result.

def __deepcopy__(self, memo):
    result = super().__deepcopy__(memo)
    result.__class__ = self.__class__
    return result

How do I get deepcopy to copy (and return) the entire Magnet object and not only a copy of the CMagnet part?

Note that the generic__deepcopy__() function copies all fields of the input object, therefore it is only the type that is wrong - the content of the copy object should be correct.

edited Jun 6, 2019 at 7:51

answered Jun 4, 2019 at 16:10

Leon

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2 Comments

zeus300 Over a year ago

Perfect, it works. You are right, it was only the type that was wrong, the methods have been there all the time. The second line in the example you provide should read result = super().__deepcopy__(memo) Providing all the material to recreate the problem, even a minimal set, is not easy with boost python, as it requires quite some of infrastructure to set the whole thing up. But luckily the information provided was sufficient after all.

Leon Over a year ago

@zeus300 Fixed the second line. Thanks.

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