A minimal CMake boost::python example for MacOS

A minimal boost::python "Hello World!" example in the form of a chello_ext Python module.

For some reason it has been hard to find something on the internet that actually compiles, links and is loadable as a Python module on macOS. See the list of potential stumbling blocks in the 'Learnings' section below.

macOS prerequisites

Make sure you have homebrew installed, then:

    brew install boost-python

For Python3:

    brew install boost-python --with-python3

CMake prerequisites

Make sure that you have matching Python versions for the calls to find_package for PythonLibs and Boost in your CMakeLists.txt.

For Python 3:

    find_package(PythonLibs 3.5 REQUIRED)
    ...
    find_package(Boost COMPONENTS python3 REQUIRED)
    ...

For Python 2:

    find_package(PythonLibs 2.7 REQUIRED)
    ...
    find_package(Boost COMPONENTS python REQUIRED)
    ...

Testing it out

$ git clone git@github.com:mkirchner/boost-python-cmake-macos-helloworld.git
Cloning into 'boost-python-cmake-macos-helloworld'...
...
Receiving objects: 100% (3/3), done.

$ cd boost-python-cmake-macos-helloworld

$ mkdir build

$ cd build

$ cmake ..
-- The C compiler identification is AppleClang 8.0.0.8000038
-- The CXX compiler identification is AppleClang 8.0.0.8000038
...
-- Configuring done
-- Generating done
-- Build files have been written to: /your/path/boost-python-cmake-macos-helloworld/build

$ make
Scanning dependencies of target chello
[ 25%] Building CXX object CMakeFiles/chello.dir/chello.cpp.o
[ 50%] Linking CXX shared library libchello.dylib
[ 50%] Built target chello
Scanning dependencies of target chello_ext
[ 75%] Building CXX object CMakeFiles/chello_ext.dir/chello_ext.cpp.o
[100%] Linking CXX shared library chello_ext.so
[100%] Built target chello_ext

$ python3
Python 3.5.2 (default, Sep 28 2016, 18:08:09) 
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import chello_ext
>>> print(chello_ext.greet())
Hello, world!
>>>

Learnings

The name of the module in the BOOST_PYTHON_MODULE(name) declaration must exactly match the name of the shared library. Hence, we need to explicitly disallow the standard lib... prefix that CMake uses for shared libs:
```
  set_target_properties(chello_ext PROPERTIES PREFIX "")
```
Python will only search for shared libraries that have a .so extension. Consequently, we explicitly need to define the appropriate suffix to avoid the macOS .dylib standard:
```
  set_target_properties(chello_ext PROPERTIES SUFFIX ".so")
```
Most of the examples on the internet lack the reference to the Python libraries in the link command.
```
  target_link_libraries(chello_ext ${Boost_LIBRARIES} ${PYTHON_LIBRARIES} chello)
```