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Py_Initialize undefined reference

I want to add python functions in C++ code.
I made a GUI in gtk (on the Raspberry PI) and now I want to work with a camera module which is easy to handle in python. (I want to start a video directly when I push a button.)
So I included the file Python.h
#include <python3.4m/Python.h>
#include <python3.4m/pythonrun.h>
then I thought it should work, but when I try to compile Py_Initialize()
I get the error:
undefined reference to Py_Initialize.
I think this is strange because, when I type in, there came the selection for Py_Initialize.

In terms of headers you should be fine, since it compiled but failed at linking.
Now you need to link against the Python libraries. The way this is done largely depends on what toolchain you are using.

Maybe you can see my answer in another question:
if with python 3.x installed, maybe this command can work:
g++ hw.cpp `/usr/bin/python3-config --cflags` `/usr/python3-config --ldflags`
By the way, you should check you gcc and python version.
As I know, if gcc version is 5.4 and python version is 3.7, it doesn't work.(python 3.5 >is work)
When you run /usr/bin/python3-config --cflags, in fact, it is the compile option.

Set the python include folder and it static lib on gcc command line and put the python dynamic lib on LD_LIBRARY_PATH. Before Py_Initialize(), do not forget to set python home with Py_SetPythonHome(). These steps must be sufficient for your code compile and run.

Linking against libgfortran and libstdc++ at the same time, in a python extension (.so)

I am having trouble building my own python extension. Building this code worked before (on Debian 7 Wheezy), but is now failing (on Ubuntu 15.04 Vivid).
The modules in question appear to link correctly, but I get an error on import. I have tried two linker lines, one with g++ (which gives me a missing symbol error for a fortran runtime function), and the other with gfortran (which gives me a missing symbol error for a runtime vtable.)
The module uses:
some polymorphic C++ code written by me,
a FORTRAN (90) routine from stripack, linked to C via a header file written by me using yolinux's guide,
a Cython file to expose some python entry points to the routines.
Thus, it needs to be linked with both the C++ standard libraries and the FORTRAN runtime, and be built into a monolithic shared object file.
I use a python build manager that I wrote myself to solve this problem: the currently published version generates the following linker line:
gfortran -fno-strict-aliasing -fPIC -pthread -shared \
-Wl,-O1 -Wl,-Bsymbolic-functions -lc -lstdc++ cpp1.o cpp2.o f90.o pyx.o -o \
module.so
(with some files omitted and paths shortened.)
This yields the following import error:
ImportError: module.so: undefined symbol: _ZTVN10__cxxabiv117__class_type_infoE
Which is apparently a reference to "vtable for __cxxabiv1::__class_type_info". This means, I think, that the -lstdc++ entry in the linker line is not doing its job correctly.
Similarly, I have tried to modify my build system to generate a g++ line linking against libgfortran like so:
c++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -lgfortran cpp1.o cpp2.o \
f90.o pyx.o -o module.so
Again, this gives me a missing symbol error:
ImportError: module.so: undefined symbol: _gfortran_st_write_done
I have also experimented with static linking, which gives an error at link time itself (like "unable to move symbol"), and initially I thought that the runtimes may be split over separate files and tried -lfoo for many foo. However, I've checked the libgfortran.so file on the linker path with scanelf, and it does contain the relevant symbol.
This method of linking used to work (on Debian Wheezy, also tested on Mac OS X 10.7 a long time ago.) I am struggling to understand how it can have broken on the latest ubuntu (with a newer GCC, 4.9).
Any ideas, theories and tests to help debug/solve this would be much appreciated.

Thank you to #Marc Glisse for your comment: the order of the arguments is indeed wrong.
If the original gfortran line is modified so that the -lc and -lstdc++ terms are last, the ImportError goes away. Thank you!

communicate between python and C++

I want to create a python module which can have its functions called from a C++ class and call c++ functions from that class
i have looked at boost however it hasn't seemed to make any sense
it refers to a shared library (which i have no idea how to create) and i cant fallow the code they use in examples (it seems very confusing)
here is their hello world tutorial
(http://www.boost.org/doc/libs/1_55_0b1/libs/python/doc/tutorial/doc/html/index.html#python.quickstart)
Following C/C++ tradition, let's start with the "hello, world". A C++ Function:
char const* greet()
{
return "hello, world";
}
can be exposed to Python by writing a Boost.Python wrapper:
include <boost/python.hpp>
BOOST_PYTHON_MODULE(hello_ext)
{
using namespace boost::python;
def("greet", greet);
}
That's it. We're done. We can now build this as a shared library. The resulting DLL is now
visible to Python. Here's a sample Python session:
>>> import hello_ext
>>> print hello_ext.greet()
hello, world
Next stop... Building your Hello World module from start to finish...
could someone please help explain what is being done and most of all how python knows about the C++ file

Python does not know about the C++ file, it will only be aware of the extension module that is compiled from the C++ file. This extension module is an object file, called a shared library. This file has an interface that looks to Python as if it was a normal Python module.
This object file will only exist after you tell a compiler to compile the C++ file and link it with all the libraries it needs. Of course, the first library needed is Boost.Python itself, which must be available on the system where you are compiling.
You can tell Python to compile the C++ file for you, so that you do not need to mess with the compiler and its library flags. In order to do so, you need a file called setup.py where you use the Setuptools library or the standard Distutils to define how your other Python modules are to be installed on the system. One of the steps for installing is compiling all extension modules, called the build_ext phase.
Let us imagine you have the following directories and files:
hello-world/
├── hello_ext.cpp
└── setup.py
The content of setup.py is:
from distutils.core import setup
from distutils.extension import Extension
hello_ext = Extension(
'hello_ext',
sources=['hello_ext.cpp'],
include_dirs=['/opt/local/include'],
libraries=['boost_python-mt'],
library_dirs=['/opt/local/lib'])
setup(
name='hello-world',
version='0.1',
ext_modules=[hello_ext])
As you can see, we are telling Python there is an Extension we want to compile, where the source file is, and where the included libraries are to be found. This is system-dependent. The example shown here is for a Mac OS X system, where Boost libraries were installed via MacPorts.
The content of hello_ext.cpp is as shown in the tutorial, but take care to reorder things so that the BOOST_PYTHON_MODULE macro comes after the definitions of whatever must be exported to Python:
#include <boost/python.hpp>
char const* greet()
{
return "hello, world";
}
BOOST_PYTHON_MODULE(hello_ext)
{
using namespace boost::python;
def("greet", greet);
}
You can then tell Python to compile and link for you by executing the following on the command line:
$ python setup.py build_ext --inplace
running build_ext
building 'hello_ext' extension
/usr/bin/clang -fno-strict-aliasing -fno-common -dynamic -pipe -Os -fwrapv -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/opt/local/include -I/opt/local/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c hello_ext.cpp -o build/temp.macosx-10.9-x86_64-2.7/hello_ext.o
/usr/bin/clang++ -bundle -undefined dynamic_lookup -L/opt/local/lib -Wl,-headerpad_max_install_names -L/opt/local/lib/db46 build/temp.macosx-10.9-x86_64-2.7/hello_ext.o -L/opt/local/lib -lboost_python-mt -o ./hello_ext.so
(The --inplace flag tells Python to leave the products of compilation right next to the source files. The default is to move them to a build directory, to keep the source directory clean.)
After that, you will find a new file called hello_ext.dll (or hello_ext.so on Unix) on the hello-world directory. If you start a Python interpreter in that directory, you will be able to import the module hello_ext and use the function greet, as shown in the Boost tutorial.

Python is an interpreted language. This means that it needs a virtual machine to execute the statements. For example, if it encounters a = 5, python (or rather the virtual machine that interprets your python code), will create an object in memory that holds some information and the value 5 and will make sure that any following reference to a will find the object. Same goes for more complex statements like input, on these commands, the virtual machine will trigger a hard coded routine which will do a lot of work under the hood before returning back to read the next piece of python code. So far, so good.
About modules. When issuing the import statement, python will look for the specified module name into its path. This is usually a .py file containing only pure python code to interpret. But that can also be a .pyd file, containing compiled routines that python can use like an executable would do with a shared library. This file contains symbols and entry points so that when the interpreter finds a special method name like mymodule.mymethod() it knows where to find the routine to execute and runs it.
However, these routines have to conform to a specific interface, and that's why it is not straightforward to expose C/C++ functions to python. The most obvious problem is that python int is not a C int, not a short, not even a long. It's a special structure that holds a lot more information like how often the variable is referenced (to be able to free memory for variables that are not referenced anymore), the type of the value it holds, etc. Of course, a typical C/C++ library doesn't work with these complex types, but uses vanilla int, float, char* and other nice plain types. So one has to translate the necessary python values to simple C types that can be understood by the library, and convert back the potential results delivered by the library into a format usable by python's virtual machine. This is what is called the wrapper. The wrapper also has to take care of funny things like reference counts, memory management on the heap, initialization and finalization, and other monkeys. See some examples to get an idea of how such code can look like. This is not extremely complicated, but still some work.
Now you get an idea of all the hard work done under the hood by the Python.Boost library (or other wrapping tools for that matters) when calling the ridiculously simple def("greet", greet);.

python c++ extension: symbol not defined error

I have a working c++ code that I want to wrap into a python module on Windows XP and Python 2.7. I have never done this before, so I looked into swig and distutils.
I created an interface file and a setup.py and compiled using
python setup.py build_ext -c mingw32
The script creates a module_wrap.cpp from my module.i and module.cpp file, and then creates a module_wrap.o and a module.o. The creation of module.o creates a bunch of Warnings for unused variables and deprecated char*, but it seems to work. Because the C++-code is not mine, I don't really want to get into these right now.
The last step is executing
g++.exe -shared -s build\temp.win32-2.7\Release\module_wrap.o build\temp.win32-2.7\Release\module.o build\temp.win32-2.7\Release\_module.def -LC:\Programme\Python27\libs -LC:\Programme\Python27\PCbuild -lpython27 -o build\lib.win32-2.7\_module.pyd
I get
Cannot export init_module: symbol not defined
error: command 'g++' failed with exit status 1
I googled a lot to this now, and I just can not find a solution to this problem. The previously created _module.def seems to try to export this init since it contains
LIBRARY _module.pyd
EXPORTS
init_module
Obviously this doesn't work, but I have no idea why. Can anyone help me out here?

I figured it out. The problem was the (not posted) interface file module.i for swig. There I named the module %module usemodule, whereas in the setup.py i named the module name=module. This way swig created an init_function, that did not match the name the created module was expecting it. In the end: just a typo...
Thanks for your support nevertheless!

Can Cython compile to an EXE?

I know what Cythons purpose is. It's to write compilable C extensions in a Python-like language in order to produce speedups in your code. What I would like to know (and can't seem to find using my google-fu) is if Cython can somehow compile into an executable format since it already seems to break python code down into C.
I already use Py2Exe, which is just a packager, but am interested in using this to compile down to something that is a little harder to unpack (Anything packed using Py2EXE can basically just be extracted using 7zip which I do not want)
It seems if this is not possible my next alternative would just be to compile all my code and load it as a module and then package that using py2exe at least getting most of my code into compiled form, right?

Here's the wiki page on embedding cython
Assuming you installed python to C:\Python31 and you want to use Microsoft Compiler.
smalltest1.py - is the file you want to compile.
test.exe - name of the executable.
You need to set the environmental variables for cl.
C:\Python31\python.exe C:\Python31\Scripts\cython.py smalltest1.py --embed
cl.exe /nologo /Ox /MD /W3 /GS- /DNDEBUG -Ic:\Python31\include -Ic:\Python31\PC /Tcsmalltest1.c /link /OUT:"test.exe" /SUBSYSTEM:CONSOLE /MACHINE:X86 /LIBPATH:c:\Python31\libs /LIBPATH:c:\Python31\PCbuild

In principal it appears to be possible to do something like what you want, according to the Embedding Pyrex HOWTO. (Pyrex is effectively a previous generation of Cython.)
Hmm... that name suggests a better search than I first tried: "embedding cython" leads to this page which sounds like what you want.

I have successfully used the Cython & gcc to convert the *.py file to *.exe, with below batch file:
# build.bat
set PROJECT_NAME=test
set PYTHON_DIR=C:\python27
%PYTHON_DIR%\python -m cython --embed -o %PROJECT_NAME%.c %PROJECT_NAME%.py
gcc -Os -I %PYTHON_DIR%\include -o %PROJECT_NAME%.exe %PROJECT_NAME%.c -lpython27 -lm -L %PYTHON_DIR%\libs

Aftershock's answer is good, what I want to say is about run app without console. Most like Aftershock's answer, if you want your application run without console, two points are important:
Replace all the main() function in the ".c" file made by cython --embed with wmain()
Add /subsystem:windows /entry:wmainCRTStartup to the end of cl.exe ... command