boost :: python导出自定义异常 [英] boost::python Export Custom Exception

问题描述

我目前正在使用Boost.Python编写Python的C ++扩展。此扩展中的函数可能会生成一个包含错误信息的异常（不仅仅是一个描述发生了什么事的人类可读的字符串）。我希望我可以导出这个异常到Python，所以我可以抓住它，并做一些额外的信息。

例如：

  import my_cpp_module 
 try：
 my_cpp_module.my_cpp_function（）
除了my_cpp_module.MyCPPException，e：
 print e。 my_extra_data 
  

不幸的是Boost.Python似乎翻译了所有C ++异常（即 std :: exception ）转换为 RuntimeError 。我意识到Boost.Python允许一个实现自定义异常翻译，但是，需要使用 PyErr_SetObject ，它需要一个 PyObject * （对于异常的类型）和 PyObject * （对于异常的值） - 我都不知道如何从我的Boost.Python类。也许有一种方法（这将是伟大的），我只是还没有发现。

解决方案

解决方案是创建一个自定义的C ++异常，你的异常类像任何正常的C ++类

  class MyCPPException：public std :: exception {...} 
  

诀窍是，所有boost :: python :: class_实例都持有对对象类型的引用， （）函数。你可以得到这个注册类与boost :: python像这样：

  class_< MyCPPException> myCPPExceptionClass（MyCPPException...）; 
 PyObject * myCPPExceptionType = myCPPExceptionClass.ptr（）; 
 register_exception_translator< MyCPPException>（& translateFunc）; 
  

最后，当您将C ++异常转换为Python异常时， / p>

  void translate（MyCPPException const& e）
 {
 PyErr_SetObject（myCPPExceptionType，boost :: python： ：object（e）.ptr（））; 
} 
  

这是一个完整的工作示例：

  #include< boost / python.hpp> 
 #include< assert.h> 
 #include< iostream> 
 
类MyCPPException：public std :: exception 
 {
 private：
 std :: string message; 
 std :: string extraData; 
 public：
 MyCPPException（std :: string message，std :: string extraData）
 {
 this-> message = message; 
 this-> extraData = extraData; 
} 
 const char * what（）const throw（）
 {
 return this-> message.c_str（）; 
} 
〜MyCPPException（）throw（）
 {
} 
 std :: string getMessage（）
 {
 return this-> ;信息; 
} 
 std :: string getExtraData（）
 {
 return this-> extraData; 
} 
}; 
 
 void my_cpp_function（bool throwException）
 {
 std :: cout< 调用C ++函数。 << std :: endl; 
 if（throwException）
 {
 throw MyCPPException（根据请求抛出异常，
这是额外的数据。 
} 
} 
 
 PyObject * myCPPExceptionType = NULL; 
 
 void translateMyCPPException（MyCPPException const& e）
 {
 assert（myCPPExceptionType！ 
 boost :: python :: object pythonExceptionInstance（e）; 
 PyErr_SetObject（myCPPExceptionType，pythonExceptionInstance.ptr（））; 
} 
 
 BOOST_PYTHON_MODULE（my_cpp_extension）
 {
 boost :: python :: class_< MyCPPException> 
 myCPPExceptionClass（MyCPPException，
 boost :: python :: init< std :: string，std :: string>（））; 
 myCPPExceptionClass.add_property（message，& MyCPPException :: getMessage）
 .add_property（extra_data，& MyCPPException :: getExtraData）; 
 myCPPExceptionType = myCPPExceptionClass.ptr（）; 
 boost :: python :: register_exception_translator< MyCPPException> 
（& translateMyCPPException）; 
 boost :: python :: def（my_cpp_function，& my_cpp_function）; 
} 
  

这是调用扩展的Python代码：

import my_cpp_extension
try：
my_cpp_extension.my_cpp_function（False）
print'这行应该没有异常应抛出。
except my_cpp_extension.MyCPPException，e：
print'Message：'，e.message
print'Extra data：'，e.extra_data

try：
my_cpp_extension.my_cpp_function（True）
print（'这行不应该被作为一个异常应该是'+
'现在抛出'。

p >

I am currently writing a C++ extension for Python using Boost.Python. A function in this extension may generate an exception containing information about the error (beyond just a human-readable string describing what happened). I was hoping I could export this exception to Python so I could catch it and do something with the extra information.

For example:

import my_cpp_module
try:
    my_cpp_module.my_cpp_function()
except my_cpp_module.MyCPPException, e:
    print e.my_extra_data

Unfortunately Boost.Python seems to translate all C++ exceptions (that are subclasses of std::exception) into RuntimeError. I realize that Boost.Python allows one to implement custom exception translation however, one needs to use PyErr_SetObject which takes a PyObject* (for the exception's type) and a PyObject* (for the exception's value)--neither of which I know how to get from my Boost.Python classes. Perhaps there is a way (which would be great) that I simply have not found yet. Otherwise does anyone know how to export a custom C++ exception so that I may catch it in Python?

解决方案

The solution is to create your exception class like any normal C++ class

class MyCPPException : public std::exception {...}

The trick is that all boost::python::class_ instances hold a reference to the object's type which is accessible through their ptr() function. You can get this as you register the class with boost::python like so:

class_<MyCPPException> myCPPExceptionClass("MyCPPException"...);
PyObject *myCPPExceptionType=myCPPExceptionClass.ptr();
register_exception_translator<MyCPPException>(&translateFunc);

Finally, when you are translating the C++ exception to a Python exception, you do so as follows:

void translate(MyCPPException const &e)
{
    PyErr_SetObject(myCPPExceptionType, boost::python::object(e).ptr());
}

Here is a full working example:

#include <boost/python.hpp>
#include <assert.h>
#include <iostream>

class MyCPPException : public std::exception
{
private:
  std::string message;
  std::string extraData;
public:
  MyCPPException(std::string message, std::string extraData)
  {
    this->message = message;
    this->extraData = extraData;
  }
  const char *what() const throw()
  {
    return this->message.c_str();
  }
  ~MyCPPException() throw()
  {
  }
  std::string getMessage()
  {
    return this->message;
  }
  std::string getExtraData()
  {
    return this->extraData;
  }
};

void my_cpp_function(bool throwException)
{
  std::cout << "Called a C++ function." << std::endl;
  if (throwException)
    {
      throw MyCPPException("Throwing an exception as requested.",
               "This is the extra data.");
    }
}

PyObject *myCPPExceptionType = NULL;

void translateMyCPPException(MyCPPException const &e)
{
  assert(myCPPExceptionType != NULL);
  boost::python::object pythonExceptionInstance(e);
  PyErr_SetObject(myCPPExceptionType, pythonExceptionInstance.ptr());
}

BOOST_PYTHON_MODULE(my_cpp_extension)
{
  boost::python::class_<MyCPPException>
    myCPPExceptionClass("MyCPPException",
            boost::python::init<std::string, std::string>());
  myCPPExceptionClass.add_property("message", &MyCPPException::getMessage)
    .add_property("extra_data", &MyCPPException::getExtraData);
  myCPPExceptionType = myCPPExceptionClass.ptr();
  boost::python::register_exception_translator<MyCPPException>
    (&translateMyCPPException);
  boost::python::def("my_cpp_function", &my_cpp_function);
}

Here is the Python code that calls the extension:

import my_cpp_extension
try:
    my_cpp_extension.my_cpp_function(False)
    print 'This line should be reached as no exception should be thrown.'
except my_cpp_extension.MyCPPException, e:
    print 'Message:', e.message
    print 'Extra data:',e.extra_data

try:
    my_cpp_extension.my_cpp_function(True)
    print ('This line should not be reached as an exception should have been' +
       'thrown by now.')
except my_cpp_extension.MyCPPException, e:
    print 'Message:', e.message
    print 'Extra data:',e.extra_data

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