Pybind Singleton

The Problem

A singleton is defined in C++. I want to expose it to Python. Different ways of binding the Singleton all indicate that Python is creating a new instance for itself, not using the one initialized by C++.

Building and Running

This is a CMake project. CMake creates two modules, "my_lib" and "my_module", that are linked to the main executable. Build all three by running

mkdir build
cd build
cmake ..
make

Be sure to run the executable ./pybind-singleton from the build directory, as there are some relative filepaths hardcoded in.

The Solution

The extern keyword was made for this. This is explained well in These StackOverflow Answers. In short, extern tells the linker that the different modules need to reference the same underlying object.

I also needed to remove static readwrite permission, i.e.

instance = Singleton.get_instance()
instance.test = 'New Value'
print(instance.test)

changes to

Singleton.set_test_val('New Value')
print(Singleton.get_test_val())