c++ boost python list extract causing Segmentation fault

Question

I am running into a strange issue with boost python. In trying to unit test some of my code, I have found that after creating a boost::python::list and appending information to it, trying to then extract that data causes a seg fault. Below is a simple example of my problem.

main.cpp

#include <boost/python.hpp>

int main()
{
    boost::python::list testList;
    float a = 1.12;
    testList.append(a);

    // This line results in a seg fault
    float b = boost::python::extract<float>(testList[0]);
    return 0;
}

And my CMakeLists.txt

cmake_minimum_required(VERSION 2.8.3)
project(boost_python_test)

find_package(Boost REQUIRED COMPONENTS system thread python)
find_package(PythonLibs REQUIRED)

IF(Boost_FOUND)
  SET(Boost_USE_STATIC_LIBS OFF)
  SET(Boost_USE_MULTITHREADED ON)
  SET(Boost_USE_STATIC_RUNTIME OFF)
ENDIF()

set(SOURCES_DIRECTORY ${PROJECT_SOURCE_DIR}/src)

set(PROJECT_SOURCES
${SOURCES_DIRECTORY}/main.cpp
)


include_directories(${Boost_INCLUDE_DIRS} ${PYTHON_INCLUDE_DIRS})

## Declare a cpp executable
add_executable(${PROJECT_NAME}
${SOURCES_DIRECTORY}/main.cpp
)

target_link_libraries(${PROJECT_NAME} ${Boost_LIBRARIES} ${PYTHON_LIBRARIES})

The compiler output is:

-- The C compiler identification is GNU 4.8.2
-- The CXX compiler identification is GNU 4.8.2
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Boost version: 1.54.0
-- Found the following Boost libraries:
--   system
--   thread
--   python
-- Found PythonLibs: /usr/lib/x86_64-linux-gnu/libpython2.7.so (found     version "2.7.6") 
-- Configuring done
-- Generating done
-- Build files have been written to:         /home/jordan/Git/boost_python_test/build

Answer 1

You're going to have to use the Py_Initialize for this to work. Check out Embedding Python. @eacousineau is the man and the real brains behind this solution.

#include <boost/python.hpp>

int main()
{
    // Need this line for it to work
    Py_Initialize();
    boost::python::list testList;
    float a = 1.12;
    testList.append(a);

    // This line no longer results in a seg fault
    float b = boost::python::extract<float>(testList[0]);
    return 0;
}