libboost_python3.so.1.56.0: undefined symbol: PyClass_Type

Question

I'm trying to create a helloWorld module for Python 3 in C++ using the boost::python library.

Here is a CmakeList.txt:

set(Python_ADDITIONAL_VERSIONS 3.4)
find_package( PythonLibs 3.4 REQUIRED )
include_directories( ${PYTHON_INCLUDE_DIRS} )

find_package( Boost 1.56.0 EXACT COMPONENTS python3 REQUIRED )
include_directories( ${Boost_INCLUDE_DIR} )

# Define the wrapper library that wraps our library
add_library( hello SHARED main.cpp )
target_link_libraries( hello ${Boost_LIBRARIES} ${PythonLibs_LIBRARIES} )

# don't prepend wrapper library name with lib
set_target_properties( hello PROPERTIES PREFIX "" OUTPUT_NAME hello)

main.cpp

#include <boost/python.hpp>

char const* greet( )
{
    return "Hello world";
}    
BOOST_PYTHON_MODULE(mymodule)
{
    using namespace boost::python;
    def( "greet", greet );
}

I installed boost libraries from the source described here, but it does not allow me to use the boost-python3 library (have an error in Cmake). For this purpose I used

./bootstrap.sh --with-python-version=3.4 --prefix=/usr/local

instead of

./bootstrap.sh --prefix=/usr/local

to explicitly specify version of python;

As an output, we get a shared library hello.so. All seems to be ok. But...

When I try to import the library to python script script.py with content:

import hello

in terminal using command ...$ python3 script.py

I receive an error

Traceback (most recent call last):
  File "script.py", line 1, in <module>
    import hello 
ImportError: /usr/local/lib/libboost_python3.so.1.56.0: undefined symbol: PyClass_Type

The question is: How to make the boost library compatible with Python 3? There are no problems with python2. But I need python3. I also saw the page when the same error happens but it didn't help me.

My software:

• boost version 1.56.0
• Python 3.4
• cmake version 2.8.12.2
• gcc 4.8.2
• OS: Ubuntu 14.04 LTS, 64 bit

Answer 1

As noted in this answer:

PyClass_Type is is part of the Python 2 C API and not part of the Python 3 C API. Hence, the Boost.Python library was likely built against Python 2. However, it is being loaded by a Python 3 interpreter, where the PyClass_Type is not available.

The exact procedure used to produce libboost_python3.so is not presented, so I can only speculate a non clean build, such as building Boost.Python with Python2, then reconfiguring bootstrap with Python3, and then building Boost.Python with the Python2 object files. Regardless, verify a clean build of Boost.Python with Python3.

$ ./bootstrap.sh --with-python=/usr/bin/python2
...
Detecting Python version... 2.7
$ ./b2 --with-python --buildid=2 # produces libboost_python-2.so
$ ./bootstrap.sh --with-python=/usr/bin/python3 --with-python-root=/usr
...
Detecting Python version... 3.3
$ ./b2 --with-python --buildid=3noclean # produces libboost_python-3noclean.so
$ ./b2 --with-python --clean
$ ./b2 --with-python --buildid=3 # produces libboost_python-3.so

$ nm -D stage/lib/libboost_python-2.so | grep PyClass_Type
                 U PyClass_Type
$ nm -D stage/lib/libboost_python-3noclean.so | grep PyClass_Type
                 U PyClass_Type
$ nm -D stage/lib/libboost_python-3.so | grep PyClass_Type

As expected, libboost_python-2.so references the PyClass_Type symbol. Additionally, the libboost_python-3noclean.so contains a reference to PyClass_Type as it was built with libboost_python-2.so's object files. With a clean build, libboost_python-3.so should not contain a reference to PyClass_Type.