[C++/Python] 如何在C++中使用一个Python类? (Use Python-defined class in C++)

最近在做基于OpenCV的车牌识别, 其中需要用到深度学习的一些代码(Python), 所以一开始的时候开发语言选择了Python(祸患之源).

固然现在Python的速度不算太慢, 但你一定要用Python来操作图像, 实现某些算法的时候, 效率就变得非常重要. 可惜的是, Python在大多数算法实现中, 由于其循环操作实在是太慢, 导致实现的算法效率非常之低.

所以现在我要把深度学习中的一个类(分类器)转换到C++中, 在这个过程之前, 需要做一些test projects, 我主要参照的文章是: C++调用Python(3).

开发环境为 VS2012, WIN7 64.

所有代码都在最后放出, 这里先逐步讲解.

首先我们需要导入合适的头文件: Python.h, 它位于你的Python安装目录的include目录下. (2.6, 2.7版本实测通过)

使用VS的话, 可以按如下方法设置一下项目属性:

1. 调试 -> xxx属性(最后一栏) -> VC++目录 -> 右侧库目录
2. 增加C:\Python27\libs, 具体路径个人可能不同.

这样就完成了最基本的配置, 下面我们开始导入.py文件并使用它.

我们使用的.py文件非常简单, 代码如下:

 #!/usr/bin/python
 # Filename: testpy.py
 class Person:
     def sayHi(self):
         print 'hi'
 class Second:
     def invoke(self,obj):
          obj.sayHi()
 def sayhi(name):
     print 'hi',name;

注: 下述所有导入方法在导入失败时不会报错, 只会返回空指针.

第一步是导入.py文件:

1. 使用PyObject* pModule来存储导入的.py文件模块, 调用的方法是PyImport_ImportModule(path):  PyObject* pModule = PyImport_ImportModule("testpy");
2. 使用PyObject* pDict来存储导入模块中的方法字典, 调用的方法是PyModule_GetDict(module):  PyObject* pDict = PyModule_GetDict(pModule);

这样就完成了.py文件的导入.

第二步是导入已导入模块中的方法或类:

1. 获取方法, 调用的方法是PyDict_GetItemString(dict, methodName): PyObject* pFunHi = PyDict_GetItemString(pDict, "sayhi");
2. 获取类, 调用的方法同上, 注意红体部分的字符串对应于.py文件中的类/方法名:  PyObject* pClassSecond = PyDict_GetItemString(pDict, "Second");

第三步是使用导入的方法或类:

1. 使用方法, 调用PyObject_CallFunction(pFunc, "s", args)即可:  PyObject_CallFunction(pFunHi, "s", "lhb");
2. 使用类构造对象, 调用PyInstance_New(pClass, NULL, NULL)即可:  PyObject* pInstanceSecond = PyInstance_New(pClassSecond, NULL, NULL); , 注意其中的pClassSecond为第二步.2中获取的类指针
3. 使用类对象的方法, 调用PyObject_CallMethod(pInstance, methodname, "O", args)即可:  PyObject_CallMethod(pInstanceSecond, "invoke", "O", pInstancePerson);
4. 上述调用中的"s"和"O"代表的是参数列表的类型, 我们可以在 Py_BuildValue 找到所有的类型, 本文最后也附了此表.

最后不要忘记销毁这些对象:  Py_DECREF(pointer);

下面是C++的实现代码, 代码来自于我参考的博客, 略有修改.

 /*
  * test.cpp
  *  Created on: 2010-8-12
  *      Author: lihaibo
  */
 #include <C:/Python27/include/Python.h>
 #include <iostream>
 #include <string>

 int main(void) {
     Py_Initialize(); // 启动虚拟机
     if (!Py_IsInitialized())
         return -;
     // 导入模块
     PyObject* pModule = PyImport_ImportModule("testpy");
     if (!pModule) {
         printf("Cant open python file!/n");
         return -;
     }
     // 模块的字典列表
     PyObject* pDict = PyModule_GetDict(pModule);
     if (!pDict) {
         printf("Cant find dictionary./n");
         return -;
     }
     // 演示函数调用
     PyObject* pFunHi = PyDict_GetItemString(pDict, "sayhi");
     PyObject_CallFunction(pFunHi, "s", "lhb");
     Py_DECREF(pFunHi);
     // 演示构造一个Python对象，并调用Class的方法
     // 获取Second类
     PyObject* pClassSecond = PyDict_GetItemString(pDict, "Second");
     if (!pClassSecond) {
         printf("Cant find second class./n");
         return -;
     }
     //获取Person类
     PyObject* pClassPerson = PyDict_GetItemString(pDict, "Person");
     if (!pClassPerson) {
         printf("Cant find person class./n");
         return -;
     }
     //构造Second的实例
     PyObject* pInstanceSecond = PyInstance_New(pClassSecond, NULL, NULL);
     if (!pInstanceSecond) {
         printf("Cant create second instance./n");
         return -;
     }
     //构造Person的实例
     PyObject* pInstancePerson = PyInstance_New(pClassPerson, NULL, NULL);
     if (!pInstancePerson) {
         printf("Cant find person instance./n");
         return -;
     }
     //把person实例传入second的invoke方法
     PyObject_CallMethod(pInstanceSecond, "invoke", "O", pInstancePerson);
     //释放
     Py_DECREF(pInstanceSecond);
     Py_DECREF(pInstancePerson);
     Py_DECREF(pClassSecond);
     Py_DECREF(pClassPerson);
     Py_DECREF(pModule);
     Py_Finalize(); // 关闭虚拟机
     return ;
 }

类型参照:

s (string) [char *]

Convert a null-terminated C string to a Python object. If the C string pointer is NULL, None is used.

s# (string) [char *, int]

Convert a C string and its length to a Python object. If the C string pointer is NULL, the length is ignored and None is returned.

z (string or None) [char *]

Same as s.

z# (string or None) [char *, int]

Same as s#.

u (Unicode string) [Py_UNICODE *]

Convert a null-terminated buffer of Unicode (UCS-2 or UCS-4) data to a Python Unicode object. If the Unicode buffer pointer is NULL, Noneis returned.

u# (Unicode string) [Py_UNICODE *, int]

Convert a Unicode (UCS-2 or UCS-4) data buffer and its length to a Python Unicode object. If the Unicode buffer pointer is NULL, the length is ignored and None is returned.

i (integer) [int]

Convert a plain C int to a Python integer object.

b (integer) [char]

Convert a plain C char to a Python integer object.

h (integer) [short int]

Convert a plain C short int to a Python integer object.

l (integer) [long int]

Convert a C long int to a Python integer object.

B (integer) [unsigned char]

Convert a C unsigned char to a Python integer object.

H (integer) [unsigned short int]

Convert a C unsigned short int to a Python integer object.

I (integer/long) [unsigned int]

Convert a C unsigned int to a Python integer object or a Python long integer object, if it is larger than sys.maxint.

k (integer/long) [unsigned long]

Convert a C unsigned long to a Python integer object or a Python long integer object, if it is larger than sys.maxint.

L (long) [PY_LONG_LONG]

Convert a C long long to a Python long integer object. Only available on platforms that support long long.

K (long) [unsigned PY_LONG_LONG]

Convert a C unsigned long long to a Python long integer object. Only available on platforms that support unsigned long long.

n (int) [Py_ssize_t]

Convert a C Py_ssize_t to a Python integer or long integer.

New in version 2.5.

c (string of length 1) [char]

Convert a C int representing a character to a Python string of length 1.

d (float) [double]

Convert a C double to a Python floating point number.

f (float) [float]

Same as d.

D (complex) [Py_complex *]

Convert a C Py_complex structure to a Python complex number.

O (object) [PyObject *]

Pass a Python object untouched (except for its reference count, which is incremented by one). If the object passed in is a NULL pointer, it is assumed that this was caused because the call producing the argument found an error and set an exception. Therefore, Py_BuildValue()will return NULL but won’t raise an exception. If no exception has been raised yet, SystemError is set.

S (object) [PyObject *]

Same as O.

N (object) [PyObject *]

Same as O, except it doesn’t increment the reference count on the object. Useful when the object is created by a call to an object constructor in the argument list.

O& (object) [converter, anything]

Convert anything to a Python object through a converter function. The function is called with anything (which should be compatible withvoid *) as its argument and should return a “new” Python object, or NULL if an error occurred.

(items) (tuple) [matching-items]

Convert a sequence of C values to a Python tuple with the same number of items.

[items] (list) [matching-items]

Convert a sequence of C values to a Python list with the same number of items.

{items} (dictionary) [matching-items]

Convert a sequence of C values to a Python dictionary. Each pair of consecutive C values adds one item to the dictionary, serving as key and value, respectively.

If there is an error in the format string, the SystemError exception is set and NULL returned.