How can I use C++ class in Python?

Question

I have implemented a class in C++. I want to use it with Python. Please suggest step by step method and elaborate each step. Somthing like this...

class Test{
     private:
        int n;
     public:
        Test(int k){
            n=k;
        }
        void setInt(int k){
            n = k; 
        }
        int getInt(){
            return n;
        }
};

Now, in Python

>>> T1 = Test(12)
>>> T1.getInt()
12
>>> T1.setInt(32)
>>> T1.getInt()
32

Please suggest.How can I do this ? NOTE: I would like to know manual way to do that. I don't want any third party library dependency.

Just curious - why do you want to avoid 3rd parties?

Andrew Hare
– Andrew Hare

2009-03-02 15:07:04 +00:00
Commented Mar 2, 2009 at 15:07 — Andrew Hare
– Andrew Hare, Commented Mar 2, 2009 at 15:07

Johannes Schaub - litb · Accepted Answer · 2009-03-02 14:58:15Z

28

Look into Boost.Python. It's a library to write python modules with C++.

Also look into SWIG which can also handle modules for other scripting languages. I've used it in the past to write modules for my class and use them within python. Works great.

You can do it manually by using the Python/C API, writing the interface yourself. It's pretty lowlevel, but you will gain a lot of additional knowledge of how Python works behind the scene (And you will need it when you use SWIG anyway).

edited Mar 2, 2009 at 14:58

answered Mar 2, 2009 at 14:49

Johannes Schaub - litb

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2 Comments

Constantin Over a year ago

Doesn't meet "I don't want any third party library dependency" requirement.

Johannes Schaub - litb Over a year ago

You will need the python library anyway. it includes the python/c api of course.

dan-gph · Accepted Answer · 2015-10-30 11:44:26Z

13

ctypes is good. It is really easy to use, and it comes standard with Python. Unfortunately it can only talk to shared libraries (Unix) or DLLs (Windows) that have a C-style interface, which means you can't directly interface to a C++ object. But you could use a handle system where a handle refers to a particular object.

>>> getInt(h)
12

I think that is simple, easy to understand, and doesn't require extra libraries.

edited Oct 30, 2015 at 11:44

answered Mar 2, 2009 at 15:06

dan-gph

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2 Comments

Kyle Bridenstine Over a year ago

Can you please elaborate on what it means, "you cannot talk to shared libraries or DLLs, which means you couldn't directly interface to a C++ object"? What would be change behavior wise when using C++ objects in Python? Would it have a noticeable impact? I need to eventually write a binding to my C++ API in Python, Thanks in advance for any help.

dan-gph Over a year ago

@2g7hgfdr8324, you misquoted me. I didn't say "you cannot talk to shared libraries or DLLs". I said the opposite. Yes, if you can expose your C++ API as a DLL, then you can call it from Python via ctypes. Your interface will have to be plain C though, not C++.

Martí · Accepted Answer · 2022-04-24 02:45:45Z

This is a very old post, and probably the OP already found a solution to his problem, but since no one has yet provided a valid example, I will give it a try and maybe help someone like me with the same problem in hand.

Since the OP did not specify, I will restrict my answer to Windows, although a Linux solution would be straightforward. I will try to provide a minimal working example that reproduces the code in question.

Step 1) Start with the C++ code (a single file)

class Test{
     private:
        int n;
     public:
        Test(int k){
            n=k;
        }
        void setInt(int k){
            n = k;
        }
        int getInt(){
            return n;
        }
};

extern "C" 
{
    // include below each method you want to make visible outside
    __declspec(dllexport) Test* init(int k) {return new Test(k);}
    __declspec(dllexport) void setInt(Test *self, int k) {self->setInt(k);}
    __declspec(dllexport) int getInt(Test *self) {return self->getInt();}
    
    // Note: the '__declspec(dllexport)' is only necessary in Windows
}

Step 2) Compile the DLL shared library. E.g. from your working directory:

g++ -shared mytest.cpp -o libtest.dll

Step 3) Create the Python file with the wrapper:

#  mytest.py

import ctypes
import platform

# From Python 3.8 onwards, there is a reported bug in CDLL.__init__()
mode = dict(winmode=0) if platform.python_version() >= '3.8' else dict()  


lib = ctypes.CDLL('./libtest.dll', **mode)


class Test(object):
    def __init__(self, val: int):
        # Declare input and output types for each method you intend to use
        lib.init.argtypes = [ctypes.c_int]
        lib.init.restype = ctypes.c_void_p

        lib.setInt.argtypes = [ctypes.c_void_p, ctypes.c_int]
        lib.setInt.restype = ctypes.c_void_p

        lib.getInt.argtypes = [ctypes.c_void_p]
        lib.getInt.restype = ctypes.c_int

        self.obj = lib.init(val)

    def setInt(self, n):
        lib.setInt(self.obj, n)
    
    def getInt(self):
        return lib.getInt(self.obj)



T1 = Test(12)
print(T1.getInt())
T1.setInt(32)
print(T1.getInt())

Step 4) Run the code

Below are my results when I run the script with 4 different Python versions:

PS D:\Desktop\mytest> py -3.7 .\mytest.py
12
32
PS D:\Desktop\mytest> py -3.8 .\mytest.py
12
32
PS D:\Desktop\mytest> py -3.9 .\mytest.py
12
32
PS D:\Desktop\mytest> py -3.10 .\mytest.py
Traceback (most recent call last):
  File "D:\Desktop\mytest\mytest.py", line 7, in <module>
    lib = ctypes.CDLL('./libtest.dll', **mode)
  File "D:\Python\3102\lib\ctypes\__init__.py", line 374, in __init__
    self._handle = _dlopen(self._name, mode)
FileNotFoundError: Could not find module 'D:\Desktop\mytest\libtest.dll' (or one of its dependencies). Try using the full path with constructor syntax.

In the last case, I have tried with the full path and still same error.

I'm running on Windows 10 64bit and compiling with g++ 8.1.0 from MinGW64

Comments:

My answer is based in the solutions provided here, here, and here
A common problem arises when the Python and compiler (g++) architectures does not match (i.e. 32 or 64 bit)

In the same thread where you found part of the solution there's an answer saying that from Python 3.9 it only works with winmode=1. In your case it fails from v3.10, but could be related, I haven't replicated it myself
To get this working on Python 3.12 I had to call lib = ctypes.CDLL('./libtest.dll') without mode

batbrat · Accepted Answer · 2009-03-02 14:49:39Z

6

I would suggest you try SWIG or sip (KDE/PyQt).

SWIG link : http://www.swig.org/
SIP link: http://freshmeat.net/projects/python-sip/

These can be used to wrap C++ classes and provide a Pythonic interface to them.

answered Mar 2, 2009 at 14:49

batbrat

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