Python 导演 class 中的 Swig 异常

Question

我正在 Python 中使用 Swig 控制器包装一些 classes。我的每个 class 方法 return 都是一个 MyError class。发生的事情是，一旦我从我的一个 C++ 派生了一个 python class 并且我忘记了 return MyError() 对象但是我 return None "pass" 或者我忘了 return 任何东西，我的软件在 MyError class 的默认构造函数中因读取访问冲突而崩溃，我无法通过 try/catch块。

Swig 中处理此类情况的正确方法是什么？

谢谢！

Answer 1

Section 36.5.4 Exception unrolling 在 SWIG 文档中：

With directors routing method calls to Python, and proxies routing them to C++, the handling of exceptions is an important concern. By default, the directors ignore exceptions that occur during method calls that are resolved in Python. To handle such exceptions correctly, it is necessary to temporarily translate them into C++ exceptions. This can be done with the %feature("director:except") directive. The following code should suffice in most cases:

%feature("director:except") {
    if ($error != NULL) {
        throw Swig::DirectorMethodException();
    }
}

This code will check the Python error state after each method call from a director into Python, and throw a C++ exception if an error occurred. This exception can be caught in C++ to implement an error handler. Currently no information about the Python error is stored in the Swig::DirectorMethodException object, but this will likely change in the future.

It may be the case that a method call originates in Python, travels up to C++ through a proxy class, and then back into Python via a director method. If an exception occurs in Python at this point, it would be nice for that exception to find its way back to the original caller. This can be done by combining a normal %exception directive with the director:except handler shown above. Here is an example of a suitable exception handler:

%exception {
    try { $action }
    catch (Swig::DirectorException &e) { SWIG_fail; }
}

The class Swig::DirectorException used in this example is actually a base class of Swig::DirectorMethodException, so it will trap this exception. Because the Python error state is still set when Swig::DirectorMethodException is thrown, Python will register the exception as soon as the C wrapper function returns.

例子

这里有一个示例 test.i 来说明该技术：

%module test

%module(directors="1") test
%feature("director");

%feature("director:except") {
    if ($error != NULL) {
        throw Swig::DirectorMethodException();
    }
}

%exception {
    try { $action }
    catch (Swig::DirectorException &e) { SWIG_fail; }
}

%inline %{
    class MyError {
        int m_n;
    public:
        MyError(int n = 0) : m_n(n) {}
        ~MyError() {}
        int get() const { return m_n; }
    };

    class Demo {
    public:
        Demo() {}
        virtual ~Demo() {}
        virtual MyError test() { return MyError(5); }
    };

    int func(Demo* d) { return d->test().get(); }
%}

经过swiging和编译，一个demo：

>>> import test
>>> d=test.Demo()  # default class implementation
>>> test.func(d)   # Call virtual method in a C++ test function.
5

以上正常。以下覆盖不正确：

>>> class Demo2(test.Demo):  # New class
...  def test(self):         # Override virtual function
...   return 7               # But don't return a MyError object.
...
>>> d=Demo2()
>>> test.func(d)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: SWIG director type mismatch in output value of type 'MyError'

捕获了异常并返回了一个有用的异常。以下正确覆盖：

>>> class Demo2(test.Demo):
...  def test(self):
...   return test.MyError(7)
...
>>> d=Demo2()
>>> test.func(d)
7

Python 导演 class 中的 Swig 异常

Swig exception in Python director class

python

c++

exception

swig