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Wrapping C++ code with python (manually)

I have a main file(main.cpp) and a header file(nodes.hpp). The main file takes N(any positive integer) as input argument and by using the functions of header file it gives output say 'x & y' (both double).
Note:
Both main and header files are written in C++.
Both main and header files instead of using data structues as arrays,vectors, make use of Eigen Library.
I have to write a python wrapper for them, I have working knowledge of python but have never used any wrapper.
Can anybody please refer or give some notes about using python wrpper for such code?

Here are your options:
You can use ctypes, and I consider this the cleanest solution, because you convert your program to a shared library that can be called by any other software, not only Python. You, though, have to write a C-interface for your program yourself.
You can use Python C-Extension, and I consider this the worst solution, because it's very low level, and prone to memory leaks, and costs lots of time to implement one function, and is Python-version dependent. Basically this is good to start a Python interpreter inside your C++. You can create PyObjects (which is the main building block of any Python type) and deal with them insdie C/C++.
You can use SWIG, where it automatically creates the the interface that you have to create with ctypes through an interface file that you define. People say it's very good, but the documentation is not as good.
You can use Boost.Python, which is good, but it has a very ugly build system with bjam. If you can manage to bypass that, then it's even better than ctypes. I'm a big boost fan, but bjam is why I don't use this.
What I do typically is ctypes. I trust it because it emphasizes the single-reponsibility principle. The library has a job that's separate from the interface (the C-interface), which is also separate from your Python script that uses that interface and exposes "the easy functionality" to the user.

Use Boost.Python. Here is my tutorial, previously on SO Docs.
Using Boost.Python
Things are easy when you have to use a C++ library in a Python project. Just you can use Boost.
First of all here is a list of components you need:
A CMakeList.txt file, because you're going to use CMake.
The C++ files of the C++ project.
The python file - this is your python project.
Let's start with a small C++ file. Our C++ project has only one method which returns some string "This is the first try". Call it CppProject.cpp
char const *firstMethod() {
return "This is the first try.";
}
BOOST_PYTHON_MODULE(CppProject) {
boost::python::def("getTryString", firstMethod); // boost::python is the namespace
}
Have a CMakeLists.txt file a below:
cmake_minimum_required(VERSION 2.8.3)
FIND_PACKAGE(PythonInterp)
FIND_PACKAGE(PythonLibs)
FIND_PACKAGE(Boost COMPONENTS python)
INCLUDE_DIRECTORIES(${Boost_INCLUDE_DIRS} ${PYTHON_INCLUDE_DIRS})
PYTHON_ADD_MODULE(NativeLib CppProject)
FILE(COPY MyProject.py DESTINATION .) # See the whole tutorial to understand this line
By this part of the tutorial everything is so easy. you can import the library and call method in your python project. Call your python project MyProject.py.
import NativeLib
print (NativeLib.getTryString)
In order to run your project follow the instructions below:
Create a directory with the name build.
Enter into that directory.
Give the command cmake -DCMAKE_BUILD_TYPE=Release ..
make
python MyProject.py. Now, you have to see the string which the method in your C++ project returns.

Another tool for C++ wrapper generation is CLIF. Released in 2017, Google uses this for most everything these days. We no longer allow new SWIG wrappers to be written for Python internally.
It is built on top of Clang for the C++ parsing and requires relatively idiomatic modern C++ API use (unsurprisingly following Google's Style Guide) rather than any attempt to allow you to shoot yourself in the foot via SWIG's "support everything poorly" approach.

Try with official documentation:
https://docs.python.org/2/extending/extending.html
this link will provide you simple example of how to include a cpp module and use it from the python interpreter, or if this is possible try with Cython: http://cython.org/
Cython will allow you to write C-like, Python-like code which will be translated to CPP compiled and then will be easily accessible from the Python.

You can use Boost.Python
or go with the Python native interface
I would recommend Boost.Python if you already have Boost set up.

Embedding Python into C++

If this question could be worded better/needs to be split into many questions, please alert me
I need to package Python scripts in order to ship them as single-executables (ideally), or single-executables with supporting files (non-ideally).
I have seen py2app and py2exe. They do not fit my requirements, as I am looking for a single method to do this, and in the future may need to have the packaged scripts interact with the executable that they are being run from.
What is the best way to go about this? The scripts which I would be embedding may even require multiple files, which complicates matters I'm sure.
If I wanted to use an interpreter other than CPython (ie: PyPy or Stackless) in the future, would the process be different (other than API calls in the C++ executable)?
Does Python have to be installed on the computers which would be running the package, or does embedding Python mean that it is fully embedded? I saw on the Python Wiki something about Py_SetPythonHome(), which would indicate to me that it needs Python (or at least its libraries) to be installed. Am I correct?

PyInstaller with the --onefile option will put everything inside a single file, though it will take a little more time to startup. I'm afraid it's not compatible with PyPy, but it should not be so tricky to get it working using Stackless.
Good luck!

Are you sure that you need to embed the Python files?
I ask because you mention you want to package the Python files as single executables. Couldn't you install Python on the target machine and since Python scripts are executables on their own, you would only need something to kick them off. A master python script could kick off all the rest of the scripts.
Otherwise you should look in C++ what can run a Python script. Then have the master python script run all the other scripts.

Yes you can!
The easiest is with newest Python3 <OS> <arch> embeddable zip file, which contains .so/.dll and all modules.
Instructions are here: https://docs.python.org/3/extending/embedding.html
#include <Python.h>
int
main(int argc, char *argv[])
{
wchar_t *program = Py_DecodeLocale(argv[0], NULL);
if (program == NULL) {
fprintf(stderr, "Fatal error: cannot decode argv[0]\n");
exit(1);
}
Py_SetProgramName(program); /* optional but recommended */
Py_Initialize();
PyRun_SimpleString("from time import time,ctime\n"
"print('Today is', ctime(time()))\n");
Py_Finalize();
PyMem_RawFree(program);
return 0;
}
The trickier part is communication between your C++ code and Python code.
If you only have to pass a few strings back and forth, that's trivial, but if you want a very tight integration, then you'll probably need a few of these:
create a Python module in (C++ code, C ABI) and inject it -- that means Python will be able to run import yourmod; yourmod.yourfunc(...) which will run your C++ custom code and/or
resolve and call Python functions from C++ (the inverse) https://stackoverflow.com/a/3310608/705086
Single executable
If you cannot have an installer, and really do not wish to have a bunch of .DLL/.so/etc it's described e.g. here Merge DLL into EXE? and it's ABI-specific, there will be one way for Windows, another Linux, etc, and you may have to override Python's import machinery (so that it doesn't look for dll/so in the file system) -- at this point, you may as well use PyInstaller or extend/merge PyInstaller with your application. Please research https://github.com/pyinstaller/pyinstaller/tree/develop/bootloader/src and evaluate how hard that might be
PyPy
Simple embedding is possisble http://doc.pypy.org/en/latest/embedding.html but single-file embedding is not. You could hack PyPy source and build custom embeddable object, but you'd be the first to do so :)
Stackless
Should be as simple as regular CPythion

How to compile a Python package to a dll

Well, I have a Python package. I need to compile it as dll before distribute it in a way easily importable. How? You may suggest that *.pyc. But I read somewhere any *.pyc can be easily decompiled!
Update:
Follow these:
1) I wrote a python package
2) want to distribute it
3) do NOT want distribute the source
4) *.pyc is decompilable >> source can be extracted!
5) dll is standard

Write everything you want to hide in Cython, and compile it to pyd. That's as close as you can get to making compiled python code.
Also, dll is not a standard, not in Python world. They're not portable, either.

Nowadays a simple solutino exists: use Nuitka compiler as described in Nuitka User Manual
Use Case 2 - Extension Module compilation
If you want to compile a single extension module, all you have to do is this:
python -m nuitka --module some_module.py
The resulting file some_module.so can then be used instead of some_module.py.
You need to compile for each platform you want to support and write some initialization code to import so/pyd file ~~appropriate for given platform/python version etc.~~
[EDIT 2021-12]: Actually in python 3 the proper so/dll is determined automatically based on the file name (if it includes python version and platform - can't find PEP for this feature at the moment but Nuitka creates proper names for compiled modules). So for python 2.7 the library name would be something.pyd or something.so whereas for python 3 this would change to something.cp36-win32.pyd or something.cpython-36m-x86_64-linux-gnu.so (for 32bit python 3.6 on x86).
The result is not DLL as requested but Python-native compiled binary format (it is not bytecode like in pyc files; the so/pyd format cannot be easily decompiled - Nuitka compiles to machine code through C++ translation)
EDIT [2020-01]: The compiled module is prone to evaluation methods using python standard mechanisms - e.g. it can be imported as any other module and get its methods listed etc. To secure implementation from being exposed that way there is more work to be done than just compiling to a binary module.

You can use py2exe.org to convert python scripts into windows executables. Granted this will only work on windows, but it's better then nothing.

You can embed python inside C. The real trick is converting between C values and Python values. Once you've done that, though, making a DLL is pretty straightforward.
However, why do you need to make a dll? Do you need to use this from a non-python program?

Python embedding is supported in CFFI version 1.5, you can create a .dll file which can be used by a Windows C application.

I would also using Cython to generate pyd files, like Dikei wrote.
But if you really want to secure your code, you should better write the important stuff in C++. The best would be to combine both C++ and Python. The idea: you would leave the python code open for adjustments, so that you don't have to compile everything over and over again. That means, you would write the "core" in C++ (which is the most secure solution these days) and use those dll files in your python code. It really depends what kind of tool or program you are building and how you want to execute it. I create mostly an execution file (exe,app) once I finish a tool or a program, but this is more for the end user. This could be done with py2exe and py2app (both 64 bit compatible). If you implement the interpreter, the end user's machine doesn't have to have python installed on the system.
A pyd file is the same like a dll and fully supported inside python. So you can normally import your module. You can find more information about it here.
Using and generating pyd files is the fastest and easiest way to create safe and portable python code.
You could also write real dll files in C++ and import them with ctypes to use them (here a good post and here the python description of how it works)

To expand on the answer by Nick ODell
You must be on Windows for DLLs to work, they are not portable.
However the code below is cross platform and all platforms support run-times so this can be re-compiled for each platform you need it to work on.
Python does not (yet) provide an easy tool to create a dll, however you can do it in C/C++
First you will need a compiler (Windows does not have one by default) notably Cygwin, MinGW or Visual Studio.
A basic knowledge of C is also necessary (since we will be coding mainly in C).
You will also need to include the necessary headers, I will skip this so it does not become horribly long, and will assume everything is set up correctly.
For this demonstration I will print a traditional hello world:
Python code we will be converting to a DLL:
def foo(): print("hello world")
C code:
#include "Python.h" // Includes everything to use the Python-C API
int foo(void); // Declare foo
int foo(void) { // Name of our function in our DLL
Py_Initialize(); // Initialise Python
PyRun_SimpleString("print('hello world')"); // Run the Python commands
return 0; // Finish execution
}
Here is the tutorial for embedding Python. There are a few extra things that should be added here, but for brevity I have left those out.
Compile it and you should have a DLL. :)
That is not all. You will need to distribute whatever dependencies are needed, that will mean the python36.dll run-time and some other components to run the Python script.
My C coding is not perfect, so if anyone can spot any improvements please comment and I will do my best to fix the it.
It might also be possible in C# from this answer How do I call a specific Method from a Python Script in C#?, since C# can create DLLs, and you can call Python functions from C#.

You can use pyinstaller for converting the .py files into executable with all required packages into .dll format.
Step 1. pip install pyinstaller,
step 2. new python file let's name it code.py .
step 3. Write some lines of code i.e print("Hello World")
step 4. Open Command Prompt in the same location and write pyinstaller code.py hit enter. Last Step see in the same location two folders name build, dist will be created. inside dist folder there is folder code and inside that folder there is an exe file code.exe along with required .dll files.

If your only goal is to hide your source code, it is much simpler to just compile your code to an executable(use PyInstaller, for example), and use an module with readable source for communication.
NOTE: You might need more converter functions as shown in this example.
Example:
Module:
import subprocess
import codecs
def _encode_str(str):
encoded=str.encode("utf-32","surrogatepass")
return codecs.encode(encoded,"base64").replace(b"\n",b"")
def _decode_str(b64):
return codecs.decode(b64,"base64").decode("utf-32","surrogatepass")
def strlen(s:str):#return length of str;int
proc=subprocess.Popen(["path_to_your_exe.exe","strlen",_encode_str(str).decode("ascii")],stdout=subprocess.PIPE)
return int(proc.stdout.read())
def random_char_from_string(str):
proc=subprocess.Popen(["path_to_your_exe.exe","randchr",_encode_str(str).decode("ascii")],stdout=subprocess.PIPE)
return _decode_str(proc.stdout.read())
Executable:
import sys
import codecs
import random
def _encode_str(str):
encoded=str.encode("utf-32","surrogatepass")
return codecs.encode(encoded,"base64").replace(b"\n",b"")
def _decode_str(b64):
return codecs.decode(b64,"base64").decode("utf-32","surrogatepass")
command=sys.argv[1]
if command=="strlen":
s=_decode_str(sys.argv[2].encode("ascii"))
print(len(str))
if command=="randchr":
s_decode_str(sys.argv[2].encode("ascii"))
print(_encode_str(random.choice(s)).decode("ascii"))
You might also want to think about compiling different executables for different platforms, if your package isn't a windows-only package anyways.

This is my idea, it might work. I don't know, if that work or not.
1.Create your *.py files.
2.Rename them into *.pyx
3.Convert them into *.c files using Cython
4.Compile *.c into *.dll files.
But I don't recommend you because it won't work on any other platforms, except Windows.

Grab Visual Studio Express and IronPython and do it that way? You'll be in Python 2.7.6 world though.

Compiling Python to C using Cython

I'm trying to compile python source code foo.py to C using cython.
In foo.py:
print "Hello World"
The command I'm running is cython foo.py.
The problem is that when compiling foo.c using gcc, I get the error:
undefined reference to 'main'.

when converting the code from python to c (using Cython) it converts it to c code which can be compiled into a shared object.
in order to make it executable, you should add "--embed" to cython conversion command. this flag adds the 'main' function you need, so you could compile the c code into executable file.
please notice you'll need the python .so runtime libraries in order to run the exec.

Read the Cython documentation. This will also (hopefully) teach you what Cython is and what it isn't. Cython is for creating python extensions (not a general-purpose Python-to-C-compiler), which are shared objects/dlls. Dynamically loaded libraries don't have a main function like standalone programs, but compilers assume that they are ultimately linking an executable. You have to tell them otherwise via flags (-shared methinks, but again, refer to the Cython documentation) - or even better, don't compile yourself, use a setup.py for this (yet again, read the Cython documentation).

The usual way is to use distutils to compile the cython-generated file. This also gives you all the include directories you need in a portable way.

Compiling C-dll for Python OR SWIG-module creation, how to continue?

I reference this file "kbdext.c" and its headerfile listed on http://www.docdroppers.org/wiki/index.php?title=Writing_Keyloggers (the listings are at the bottom).
I've been trying to compile this into a dll for use in Python or Visual Basic, but have not succeeded. I'm not familiar with C or GCC to sort out the problems or do the dll compile correctly. (I also get an error about snprintf not being declared when doing a regular compile of all the files).
What are the steps I should do to make all functions available for other languages and external apps?
Or is it perhaps easier to use SWIG and make a python module, instead of compiling a DLL?

I've succeeded in compiling the dll with GCC, and am able to import its functions in C. I have yet to test the import in VB and Python but can't see why it would pose problems.