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我正在将一个用 C++ 编写的库包装到 Python API libwebqq

在 boost 函数中定义了一种类型。

typedef boost::function<void (std::string)> EventListener;

Python 级别可以定义“EventListener”变量回调。

C++ 级别还有一个映射结构，即适配器类中的 event_map。event_map 的键类型是 QQEvent 枚举类型，而 event_map 的值类型是包装了 EvenListener 的类“Action”。

class Action
{

    EventListener _callback;

    public:
    Action (){
        n_actions++;
    }

    Action(const EventListener & cb ){
    setCallback(cb);
    }

    virtual void operator()(std::string data) {
    _callback(data);
    }
    void setCallback(const EventListener & cb){
        _callback = cb;
    }

    virtual ~Action(){ std::cout<<"Destruct Action"<<std::endl; n_actions --; }
    static int n_actions;
};


class Adapter{

    std::map<QQEvent, Action > event_map;

public:
    Adapter();
    ~Adapter();
    void trigger( const QQEvent &event, const std::string data);
    void register_event_handler(QQEvent event, EventListener callback);
    bool is_event_registered(const QQEvent & event);
    void delete_event_handler(QQEvent event);
};

类 Adapter 中的“register_event_handler”是向相关事件注册回调函数的 API。如果事件发生，C++ 后端将调用它。但是我们需要在 python 级别实现回调。我将回调类型包装在“callback.i”中

问题是，当我在测试 python脚本中调用 register_event 时，总是会出现类型错误：

Traceback (most recent call last):
File "testwrapper.py", line 44, in <module>
worker = Worker()
File "testwrapper.py", line 28, in __init__
a.setCallback(self.on_message)
File "/home/devil/linux/libwebqq/wrappers/python/libwebqqpython.py", line 95, in setCallback
def setCallback(self, *args): return _libwebqqpython.Action_setCallback(self, *args)
TypeError: in method 'Action_setCallback', argument 2 of type 'EventListener const &'
Destruct Action

请帮助找出此类错误的根本原因并解决此问题。

4

1 回答 1

12

The problem seems to be that you haven't included any code to map from a Python callable to your EventListener class. It's not provided for free, although it's something that comes up fairly regularly, e.g. here which acted as a reference for part of this answer.

Your question has quite a lot of code that's not really relevant to the problem and not quite complete either so I've produced a minimal header file to demonstrate the problem and solution with:

#include <boost/function.hpp>
#include <string>
#include <map>

typedef boost::function<void (std::string)> EventListener;

enum QQEvent { THING };

inline std::map<QQEvent, EventListener>& table() {
  static std::map<QQEvent, EventListener> map;
  return map;
}

inline const EventListener& register_handler(const QQEvent& e, const EventListener& l) {
  return table()[e] = l;
}

inline void test(const QQEvent& e)  {
  table()[e]("Testing");
}

Given that header file a simple wrapper would be:

%module test

%{
#include "test.hh"
%}

%include "test.hh"

I also put together a bit of Python to run this with:

import test

def f(x):
  print(x)

test.register_handler(test.THING, f)
test.test(test.THING)

With this I can reproduce the error you see:

LD_LIBRARY_PATH=. python3.1 run.py
Traceback (most recent call last):
  File "run.py", line 6, in 
    test.register_handler(test.THING, f)
TypeError: in method 'register_handler', argument 2 of type 'EventListener const &'

Hopefully we're on the same page now. There's a single version of register_handler that expects an object of type EventListener (SWIG's generated proxy for the type to be precise). We're not trying to pass an EventListener in when we call that function though - it's a Python Callable instead, with not much known on the C++ side - certainly not a type match or implicitly convertible. So we need to add some glue in our interface to mush the Python type into the real C++ type.

We do that by defining an entirely new type, which only exists internally to the SWIG wrapper code (i.e. within %{ }%). The type PyCallback has one purpose: hold a reference to the real Python thing we're working with and make it look/feel like a function in C++.

Once we've added that PyCallback implementation detail (which nobody gets to see) we then need to add another overload for register_handler, which takes a PyObject* directly and constructs the PyCallback+EventListener for us. Since this only exists for the purpose of wrapping we use %inline to declare, define and wrap all within the SWIG interface file. So our interface file now looks like:

%module test

%{
#include "test.hh"

class PyCallback
{
    PyObject *func;
    PyCallback& operator=(const PyCallback&); // Not allowed
public:
    PyCallback(const PyCallback& o) : func(o.func) {
      Py_XINCREF(func);
    }
    PyCallback(PyObject *func) : func(func) {
      Py_XINCREF(this->func);
      assert(PyCallable_Check(this->func));
    }
    ~PyCallback() {
      Py_XDECREF(func);
    }
    void operator()(const std::string& s) {
      if (!func || Py_None == func || !PyCallable_Check(func))
        return;
      PyObject *args = Py_BuildValue("(s)", s.c_str());
      PyObject *result = PyObject_Call(func,args,0);
      Py_DECREF(args);
      Py_XDECREF(result);
    }
};
%}

%include "test.hh"

%inline %{
  void register_handler(const QQEvent& e, PyObject *callback) {
    register_handler(e, PyCallback(callback));
  }
%}

At this point we now have enough for the original test Python to run successfully.

It's worth noting that we could have chosen to hide the original overload of register_handler, but in this instance I prefer not to - it's more of a feature than a mistake to leave that visible because it permits you to manipulate C++ defined callbacks also, e.g. you could get/set them from the Python side and expose some core ones as global variables.

In your actual code you'll want to do:

%extend Action {
  void setCallback(PyObject *callback) {
    $self->setCallback(PyCallback(callback));
  }
}

to overload Action::setCallback, after the line %include "test.hh", instead of the %inline I used in my example to overload the free function.

Finally you might want to expose operator() of your Action class using %rename, or you could chose to expose the callback_ member using the function pointer/member function trick.

于 2012-07-17T12:41:34.960 回答