Windows 下用 C++ 调用 Python

Part.I Introduction

本文主要介绍一下如何使用 C++ 调用 Python。包括运行 python 脚本；C++ 和 Python 之间参数的相互转换和传递。

Chap.I Information

下面是笔者python 相关的目录，可参考

• python.exe 所在目录：A:\Programs\Python\Python11_4
• python311.lib 所在目录：A:\Programs\Python\Python11_4\libs
• include 所在目录：A:\Programs\Python\Python11_4\include

Chap.II 预备知识

Part.II 语法

Chap.I PyRun_SimpleString

Python 库函数 PyRun_SimpleString 可以执行字符串形式的Python代码。不过在使用 PyRun_SimpleString 之前需要先初始化（Py_Initialize()），执行完之后需要释放资源Py_Finalize()，示例：

Py_Initialize();
PyRun_SimpleString("print('Hello!')");
Py_Finalize();

Chap.II C++ / Python 变量之间的相互转换

有待总结和补充，可看下面的实例。

Part.III 实例

Chap.I 文件内容

所用到的几个文件组织结构如下

test
│  CMakeLists.txt
│  hello.py
│  main.cpp
└─build

hello.py中有两个小函数，内容如下：

def add(a,b):
    return a+b

def get_name(first):
    return "your name is {} alice".format(first)

main.cpp文件内容如下：

#include <iostream>
#include <Python.h>

using namespace std;

const int kError = -1;
const int kSuccess = 0;


/**
 * @brief  Initializes the python interpreter
 */
int pythonInit() {
    Py_Initialize();
    int ret = Py_IsInitialized();
    if (ret == 0) {
        cout << "Py_Initialize error" << endl;
        return kError;
    }
    return kSuccess;
}


/**
 * @brief  Release resources requested by Python
 */
void pythonCleanup() {
    Py_Finalize();
}


/**
 * @brief  Import module ${name} in ${pyDir}
 * @param[in] pyDir     The path of the python script
 * @param[in] name      The name of the python script
 * @return              the module object
 */
PyObject *pythonImportModule(const char *pyDir, const char *name) {
    // 引入当前路径,否则下面模块不能正常导入
    char tempPath[256] = {};
    sprintf(tempPath, "sys.path.append('%s')", pyDir);
    PyRun_SimpleString("import sys");
    //PyRun_SimpleString("sys.path.append('./')");
    PyRun_SimpleString(tempPath);
    PyRun_SimpleString("print('curr sys.path', sys.path)");

    // import ${name}
    PyObject *module = PyImport_ImportModule(name);
    if (module == nullptr) {
        PyErr_Print();
        cout << "PyImport_ImportModule '" << name << "' not found" << endl;
        return nullptr;
    }

    return module;
}


/**
 * @brief  Call 'add' function in the python script
 * @param[in] module    The name of module
 * @param[in] a         The value of a
 * @param[in] b         The value of b
 * @return              a + b
 */
int callPythonAdd(PyObject *module, int a, int b) {
    //获取模块字典属性
    PyObject *pDict = PyModule_GetDict(module);
    if (pDict == nullptr) {
        PyErr_Print();
        std::cout << "PyModule_GetDict error" << std::endl;
        return kError;
    }

    //直接获取模块中的函数
    PyObject *addFunc = PyDict_GetItemString(pDict, "add");
    if (addFunc == nullptr) {
        std::cout << "PyDict_GetItemString 'add' not found" << std::endl;
        return kError;
    }

    // 构造python 函数入参， 接收2
    // see: https://docs.python.org/zh-cn/3.7/c-api/arg.html?highlight=pyarg_parse#c.PyArg_Parse
    PyObject *pArg = Py_BuildValue("(i,i)", a, b);

    //调用函数，并得到 python 类型的返回值
    PyObject *result = PyEval_CallObject(addFunc, pArg);

    int ret = 0;
    //将python类型的返回值转换为c/c++类型
    PyArg_Parse(result, "i", &ret);
    return ret;
}


/**
 * @brief  Call 'get_name' function in the python script
 * @param[in] module    The name of module
 * @param[in] firstName The firstname
 * @param[in] outName   The fullname
 * @return              success or not
 */
int callPythonGetName(PyObject *module, std::string firstName, std::string &outName) {
    //获取模块字典属性
    PyObject *pDict = PyModule_GetDict(module);
    if (pDict == nullptr) {
        PyErr_Print();
        std::cout << "PyModule_GetDict error" << std::endl;
        return kError;
    }

    //直接获取模块中的函数
    PyObject *addFunc = PyDict_GetItemString(pDict, "get_name");
    if (addFunc == nullptr) {
        std::cout << "PyDict_GetItemString 'add' not found" << std::endl;
        return kError;
    }

    // 构造python 函数入参， 接收2
    // see: https://docs.python.org/zh-cn/3.7/c-api/arg.html?highlight=pyarg_parse#c.PyArg_Parse
    PyObject *pArg = Py_BuildValue("(s)", firstName.c_str());

    //调用函数，并得到python类型的返回值
    PyObject *result = PyEval_CallObject(addFunc, pArg);

    char *name = nullptr;
    //将python类型的返回值转换为c/c++类型
    PyArg_Parse(result, "s", &name);

    char tempStr[256] = {};
    int strLen = strlen(name);
    if (strLen > 256) {
        return kError;
    }
    strcpy(tempStr, name);
    outName = tempStr;
    return kSuccess;
}


int main() {
    pythonInit();

    //直接运行python代码
    PyRun_SimpleString("print('---------- Hello Python form C/C++ ----------')");

    PyObject *helloModule = pythonImportModule("../", "hello");    // 这里最好还是给绝对路径吧
    if (helloModule == nullptr) {
        return -1;
    }

    // call python add function
    int result = callPythonAdd(helloModule, 1, 3);
    cout << "1 + 3 = " << result << endl;

    // call python get_name function
    std::string fullName;
    callPythonGetName(helloModule, "summer", fullName);
    cout << fullName << endl;

    pythonCleanup();
}

CMakeLists.txt 文件内容等会儿说；build 是一个空文件，生成的二进制文件等放这里面。

Chap.II 基于 Visual Studio IDE

1、首先新建一个工程，将main.cpp添加进去，然后将hello.py放在和main.cpp一样的路径下。

2、将 IDE 上方『解决方案平台』设置为x64，最好将『解决方案配置』设置为Release（debug 需要*_d.lib）

3、将include添加到 C/C++ 附加包含目录中：项目右键→属性→C/C++→附加包含目录→添加 python 的 include

右键属性→链接器→常规→附加库目录→将 python 的 libs 加进去

3、将*.lib添加到附加依赖项中：项目右键→属性→链接器→输入→将python311.lib和python311_d.lib加进去。

Chap.III 基于 cmake

cmake 自动搜寻 python

CMakeLists.txt文件内容如下：

cmake_minimum_required( VERSION 3.20 )

project( test )

set( PRJ_INCLUDE_DIRS )
set( PRJ_COMPILE_FEATURES )
set( PRJ_LIBRARIES )

list( APPEND PRJ_COMPILE_FEATURES cxx_std_20 )

find_package(Python3 COMPONENTS Interpreter Development)

message( STATUS "Python3_FOUND = ${Python3_FOUND} " )
message( STATUS "Python3_INCLUDE_DIRS = ${Python3_INCLUDE_DIRS} " )
message( STATUS "Python3_LIBRARIES = ${Python3_LIBRARIES} " )

if( ${Python3_FOUND} )
    #include_directories(${Python3_INCLUDE_DIRS})
else()
    message( FATAL_ERROR "Python3 not found, please install it." )
endif()

list( APPEND PRJ_INCLUDE_DIRS ${Python3_INCLUDE_DIRS} )
list( APPEND PRJ_LIBRARIES ${Python3_LIBRARIES} )

message( STATUS "PRJ_LIBRARIES = ${PRJ_LIBRARIES} " )

add_executable( ${PROJECT_NAME}
    main.cpp
)

target_include_directories( ${PROJECT_NAME}
    PRIVATE 
        ${PRJ_INCLUDE_DIRS}
)

target_link_libraries( ${PROJECT_NAME} 
    PRIVATE 
        ${PRJ_LIBRARIES}
)

target_compile_features( ${PROJECT_NAME} 
    PRIVATE 
        ${PRJ_COMPILE_FEATURES}
)

用 CMake 编译一下

然后直接跑就可以！

---

cmake 手动设置 Python 路径

CMakeLists.txt文件内容如下：

cmake_minimum_required( VERSION 3.20 )

project( test )

set( PRJ_INCLUDE_DIRS )
set( PRJ_COMPILE_FEATURES )
set( PRJ_LIBRARIES )

list( APPEND PRJ_COMPILE_FEATURES cxx_std_20 )

set( Python3_INCLUDE_DIRS "A:/Programs/Python/Python11_4/include")
set( Python3_LIBRARIES "A:/Programs/Python/Python11_4/libs/python311.lib"
                       "A:/Programs/Python/Python11_4/libs/python311_d.lib" )


message( STATUS "Python3_INCLUDE_DIRS = ${Python3_INCLUDE_DIRS} " )
message( STATUS "Python3_LIBRARIES = ${Python3_LIBRARIES} " )


list( APPEND PRJ_INCLUDE_DIRS ${Python3_INCLUDE_DIRS} )
list( APPEND PRJ_LIBRARIES ${Python3_LIBRARIES} )

message( STATUS "PRJ_LIBRARIES = ${PRJ_LIBRARIES} " )

add_executable( ${PROJECT_NAME}
    main.cpp
)

target_include_directories( ${PROJECT_NAME}
    PRIVATE 
        ${PRJ_INCLUDE_DIRS}
)

target_link_libraries( ${PROJECT_NAME} 
    PRIVATE 
        ${PRJ_LIBRARIES}
)

target_compile_features( ${PROJECT_NAME} 
    PRIVATE 
        ${PRJ_COMPILE_FEATURES}
)

Chap.IV 运行结果

---------- Hello Python form C/C++ ----------
curr sys.path ['A:\\Programs\\Python\\Python11_4\\python311.zip', 'A:\\Programs\\Python\\Python11_4\\DLLs', 'A:\\Programs\\Python\\Python11_4\\Lib', 'A:\\aWork\\scripts\\test1\\test\\build\\Debug', 'A:\\Programs\\Python\\Python11_4', 'A:\\Programs\\Python\\Python11_4\\Lib\\site-packages', '../']
1 + 3 = 4
your name is summer alice

Part.IV 可能出现的问题

Chap.I 无法打开 python311_d.lib

无法打开 python311_d.lib 的问题：笔者使用的 python 版本是 Python 11.4，它的libs中没有python311_d.lib，只有python311.lib（因为安装的时候没有勾选安装 debug 的 lib），解决方法有三个：

1. 不使用debug模式运行程序，使用release或其他模式运行 C++ 程序
2. 找到pyconfig.h文件（一般在py_dir/include文件夹下），注释下面的内容（有点危险）

#ifdef _DEBUG
#       define Py_DEBUG
#endif

3. 安装 python 的 debug 版本库：安装程序→更改→勾选Download debug binaries (requires VS 2017 or later)

Chap.II 导入模块报错

这种情况下是没有把 python 脚本所在的路径加到sys.path里面，使用sys.path.append(your_path_xx)添加一下就可以了。

Chap.I PyEval_CallObject 调用报错

报错内容：

'PyEval_CallObjectWithKeywords': deprecated in 3.9

将PyEval_CallObject替换为PyObject_CallObject就行了。

Reference

• Linux 系统下通过 cmake 使 C++ 调用 Python