Embed / Include Python.h into C++ [Full Guide] (Python 3.9) (Windows) (Qt 5.15)

Question

When I was trying to embed a Python script into my Qt C++ program, I run into multiple problems when trying to include Python.h. The following features, I would like to provide:

• Include python.h
• Execute Python Strings
• Execute Python Scripts
• Execute Python Scripts with Arguments
• It should also work when Python is not installed on the deployed machine

Therefore I searched around the Internet to try to find a solution. And found a lot of Questions and Blogs, but non have them covered all my Problems and it still took me multiple hours and a lot of frustration. That's why I have to write down a StackOverflow entry with my full solution so it might help and might accelerate all your work :)

Answer 1

(This answer and all its code examples work also in a non-Qt environment. Only 2. and 4. are Qt specific)

1. Download and install Python https://www.python.org/downloads/release
2. Alter the .pro file of your project and add the following lines (edit for your correct python path):

INCLUDEPATH = "C:\Users\Public\AppData\Local\Programs\Python\Python39\include" 
LIBS += -L"C:\Users\Public\AppData\Local\Programs\Python\Python39\libs" -l"python39"

3. Example main.cpp code:

#include <QCoreApplication>

#pragma push_macro("slots")
#undef slots
#include <Python.h>
#pragma pop_macro("slots")

/*!
 * \brief runPy can execut a Python string
 * \param string (Python code)
 */
static void runPy(const char* string){
    Py_Initialize();
    PyRun_SimpleString(string);
    Py_Finalize();
}

/*!
 * \brief runPyScript executs a Python script
 * \param file (the path of the script)
 */
static void runPyScript(const char* file){
    FILE* fp;
    Py_Initialize();
    fp = _Py_fopen(file, "r");
    PyRun_SimpleFile(fp, file);
    Py_Finalize();
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    runPy("from time import time,ctime\n"
      "print('Today is', ctime(time()))\n");

    //uncomment the following line to run a script
    //runPyScript("test/decode.py");

    return a.exec();
}

4. Whenever you #include <Python.h> use the following code instead. (The Slots from Python will otherwise conflict with the Qt Slots

#pragma push_macro("slots")
#undef slots
#include <Python.h>
#pragma pop_macro("slots")

5. After compiling, add the python3.dll, python39.dll, as well as the DLLs and Lib Python folders to your compilation folder. You can find them in the root directory of your Python installation. This will allow you to run the embedded c++ code even when python is not installed.

With these steps, I was able to get python running in Qt with the 64 bit MinGW and MSVC compiler. Only the MSVC in debug mode got still a problem.

FURTHER:

If you want to pass arguments to the python script, you need the following function (It can be easy copy-pasted into your code):

/*!
 * \brief runPyScriptArgs executs a Python script and passes arguments
 * \param file (the path of the script)
 * \param argc amount of arguments
 * \param argv array of arguments with size of argc
 */
static void runPyScriptArgs(const char* file, int argc, char *argv[]){
    FILE* fp;
    wchar_t** wargv = new wchar_t*[argc];
    for(int i = 0; i < argc; i++)
    {
        wargv[i] = Py_DecodeLocale(argv[i], nullptr);
        if(wargv[i] == nullptr)
        {
            return;
        }
    }
    Py_SetProgramName(wargv[0]);

    Py_Initialize();
    PySys_SetArgv(argc, wargv);
    fp = _Py_fopen(file, "r");
    PyRun_SimpleFile(fp, file);
    Py_Finalize();

    for(int i = 0; i < argc; i++)
    {
        PyMem_RawFree(wargv[i]);
        wargv[i] = nullptr;
    }

    delete[] wargv;
    wargv = nullptr;
}

To use this function, call it like this (For example in your main):

    int py_argc = 2;
    char* py_argv[py_argc];

    py_argv[0] = "Progamm";
    py_argv[1] = "Hello";

    runPyScriptArgs("test/test.py", py_argc, py_argv);

Together with the test.py script in the test folder:

import sys

if len(sys.argv) != 2:
  sys.exit("Not enough args")

ca_one = str(sys.argv[0])
ca_two = str(sys.argv[1])
print ("My command line args are " + ca_one + " and " + ca_two)

you get the following output:

My command line args are Progamm and Hello