c++调用python时参数传递和返回值解析-创新互联

通过python接口在cpp中调用python函数时，构建传参和解析返回值都会用到python内的变量类型和c++的变量类型之间的转换。
返回值是numpy等复杂结构的数据时，可以通过先转换为list等类型再返回，或者单独构建一个函数，获取numpy内某个位置的变量的函数。

下面几个基础类型的返回值解析：

#python
#返回一个参数
def returnInt(a,b):
    return a+b
#返回两个相同类型的参数
def returnTwoInt():
    return 1,2
#返回字符串
def returnString():
    srt = 'hello world'
    return srt
#返回list
def returnList():
	para = [1, 2, 3, 4]
    return 1,para.tolist()
#返回两个不同类型的参数
def returnTwo():
	para = [1, 2, 3, 4]
    return 1,para.tolist()
#返回矩阵
def returnMat():
    img = cv2.imread('test1.bmp',cv2.IMREAD_GRAYSCALE)
    return img

//调用python函数，返回的数据统一为PyObject类型，需要根据实际类型进行解析
	PyObject* pReturn = PyObject_CallObject(pFunc, pArgs);
	//当不需要传参时，可以将参数位置为空
	PyObject* pReturn = PyObject_CallObject(pFunc,NULL);

PyObject为结构体，参数ob_type->tp_name 为数据的类型

//不同类型参数传递
	PyObject* p = PyTuple_New(4);
	for (int i = 0; i< 4; i++)
		PyTuple_SetItem(p, i, Py_BuildValue("i", i));
	PyObject* pArgs = PyTuple_New(2);
	PyTuple_SetItem(pArgs, 0,Py_BuildValue ("i",1));
	PyTuple_SetItem(pArgs, 0,Py_BuildValue ("O",p));
	
//返回为list
	double temp;
	int i_size = PyList_Size(pReturn); 
	for (int i = 0; i< i_size; ++i)
	{PyArg_Parse(PyList_GetItem(pReturn, i), "d", &temp);
		std::cout<< "return result is "<< (temp)<< std::endl;
	}
//返回为 int 和 list 两个不同类型的参数
	PyArg_Parse(PyTuple_GetItem(pReturn, 0), "d", &temp);
	std::cout<< "temp is "<< (temp)<< std::endl;
	PyObject* pReturnlist;
	PyArg_Parse(PyTuple_GetItem(pReturn, 1), "O", &pReturnlist);
	i_size = PyList_Size(pReturnli); 
	for (int i = 0; i< i_size; ++i)
	{PyArg_Parse(PyList_GetItem(pReturnli, i), "d", &temp);
		std::cout<< "return result is "<< (temp)<< std::endl;
	}
//返回单独一个变量
	//构建输入
	PyObject* pArgs = PyTuple_New(2);
	PyTuple_SetItem(pArgs, 0, Py_BuildValue("i", 1));
	PyTuple_SetItem(pArgs, 1, Py_BuildValue("i", 1));
	PyObject* pReturn2 = PyObject_CallObject(pFunc, pArgs);
	//解析
	int nResult1;	//解析为int
	PyArg_Parse(pReturn2, "i", &nResult1);
	std::cout<< "add="<< nResult1<< std::endl;
	double nResultd;	//解析为double
	PyArg_Parse(pReturn2, "d", &nResultd);
	std::cout<< "add="<< nResultd<< std::endl;
//返回字符串
	//解析方法1：
	PyObject* repr = PyObject_Repr(pReturn);
	PyObject* str = PyUnicode_AsEncodedString(repr, "utf-8", "strict");
	char* result = PyBytes_AsString(str);
	std::cout<< result<< std::endl;
	//解析方法2：
	char* pstr = NULL;
	PyArg_Parse(pReturn, "s", &pstr);
	std::cout<< pstr<< std::endl;

//返回多个变量
	//循环解析
	int nResult;
	i_size = PyTuple_Size(pReturn); 
	for (int i = 0; i< i_size; ++i)
	{PyArg_Parse(PyTuple_GetItem(pReturn, i), "i", &nResult);
		std::cout<< "return result is "<< (nResult)<< std::endl;
	}
	//直接解析
	int nResult1;
	int nResult2;
	PyArg_ParseTuple(pReturn,"i|i", &nResult1, &nResult2);   
	std::cout<< "return  "<< nResult5<< ' '<< nResult6<< std::endl;
//解析二维矩阵
	//获得序列长度，即行数
	Py_ssize_t rsize = PyObject_Size(pReturn);
	std::cout<< rsize<< std::endl;
	//转换到迭代器，用于提取数据
	PyObject* iter = PyObject_GetIter(pReturn5);
	while (true)
	{//获取矩阵的第一行
		PyObject* next = PyIter_Next(iter);
		if (!next) {// nothing left in the iterator
			break;
		}
		if (!PyList_Check(next))
		{// error, we were expecting a list value }
		//获得当前行的数据数量
		Py_ssize_t foosize = PyObject_Size(next);
		std::cout<< foosize<< std::endl;
		//转换到迭代器，用于获取数据
		PyObject* iter2 = PyObject_GetIter(next);
		while (true)
		{	PyObject* next2 = PyIter_Next(iter2);
			if (!next2)
				break;
			if (!PyFloat_Check(next2))
			 {// error, we were expecting a floating point value}
			double foo = PyFloat_AsDouble(next2);
			std::cout<< foo<< " ";
		}
		std::cout<< std::endl;
	}

python手册，接口的使用示例等
https://docs.python.org/3/

//解析常用字段：
	// b<=>char 0-255的那个char
	// c<=>char 单个字符
	// h<=>short int
	// l<=>long int
	// f<=>float
	// d<=>double
	// s<=>char*
	// O<=>PyObject

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