Python Capsules are useful for passing C pointers between different Python modules.  In particular, one can encapsulate a C function pointer in one module and unpack and call it in another module.

To begin, suppose we have two Boost.Python modules:

1. Bird, which has methods setQuack() and callQuack().
2. CppDuck, which has a method getQuack().

Our goal will be to get a function pointer with CppDuck.getQuack(), set it in another module in Bird.setQuack(), and finally call it with Bird.callQuack().

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