如何防止 Python 退回到 C++ boost::python::error_already_set？

Question

我需要在问题之前给出一些背景信息。请多多包涵。使用 boost::python 我向 Python 公开了一些异常类型，比如MyExceptionType。我有一个boomTest，我将它暴露给 Python 以检查它是否有效。 Python 调用boomTest 并正确处理MyExceptionType，到目前为止一切顺利。这是 C++ 的一面：

static void boomTest() {
    throw MyExceptionType("Smoked too many Cohibas!");
}

static PyObject *myExceptionPtr = NULL;
static void translate(MyExceptionType const &exception) {
    assert(myExceptionPtr != NULL);
    boost::python::object pythonExceptionInstance(exception);
    PyErr_SetObject(myExceptionPtr, pythonExceptionInstance.ptr());
}

BOOST_PYTHON_MODULE(test) {
    class_<MyExceptionType> myException("MyExceptionType", no_init);
    myException.add_property("message", &MyExceptionType::what);
    myExceptionPtr = myException.ptr();
    register_exception_translator<MyExceptionType>(&translate);
}

这就是 Python 的一面：

import sys

import example
reload(example)
from example import MyExceptionType, boomTest

def tryBoomTest():
    try:
        boomTest()

    except MyExceptionType as ex:
        print 'Success! MyExceptionType gracefully handled:' \
            '\n message="%s"' % ex.message
    except:
        print 'Caught unhandled exception: %s "%s"' % (sys.exc_info()[0], sys.exc_info()[1])

现在事情变得有点棘手，因为在一个真实的用例中，我有一个从 C++ Boost（非 Python）线程到 Python 的回调，如下所示：

# this is a Python callback invoked from a C++ boost non-Python thread
def handle(future):
    try:
        # future.get() throws MyExceptionType if there was a cluster exception
        "Cluster response received with value: %s" % future.get()

    except MyExceptionType as ex:
        print 'Success! MyExceptionType gracefully handled:' \
            '\n message="%s"' % ex.message

现在是 OP：

当future.get() 调用抛出一个供Python 处理的MyExceptionType 时，为什么我的C++ 回调触发器会得到一个boost::python::error_already_set 异常？我怀疑这种行为是由于异常是在 C++（非 Python）线程中引发的......

需要什么来强制 Python 像开头的示例那样处理异常？

我已经尝试在 C++ 的回调触发器中执行以下操作：

void callbackTrigger() {
    try {
        pythonHandle_(getFuture());
    }
    // why do I get this???
    catch (boost::python::error_already_set&) {
        // this doesn't help, Python doesn't still handle MyExceptionType
        boost::python::handle_exception();
    }
}

Answer 1

我验证了我的理论，即 Python 不喜欢处理抛出的异常（即使在 Python 代码中），而是作为外来 C++ 线程的一部分执行。因此，我构建了这个纯 Python 包装器，以便能够处理 Python 主线程中的回调。

import example
reload(example)
from example import MyExceptionType

condition = threading.Condition()
futures = []

# Actual handle executed by the main Python THREAD
# __after__ submitting all the jobs to the Cluster
# N is the number of jobs that were submitted to the Cluster
def handle(N):
    while (N > 0):
        condition.acquire()
        try:
            # wait for max of a second to prevent this thread waiting indefinitely
            # when it misses notify while being busy processing responses
            condition.wait(1.0)
            for future in futures:
                try:
                    print 'callback received! the response is:\n %s' % future.get()

                except MyExceptionType as ex:
                    print 'MyExceptionType gracefully handled:' \
                          '\n message="%s"' % ex.message
                except:
                    print 'Caught unhandled exception: %s "%s"' % (sys.exc_info()[0], sys.exc_info()[1])
        finally:
            N -= len(futures)
            del(futures[:])
            condition.release()

# callback called from a C++ boost THREAD
def callback(future):
    condition.acquire()
    try:
        # do not consume the future here, rather let the main 
        # Python thread deal with it ...
        futures.append(future)
        condition.notify()
    finally:
        condition.release()

这并不理想，但它可以工作并且输出正确：

registering callback 1 ...
registering callback 2 ...
registering callback 3 ...
registering callback 4 ...
registering callback 5 ...
MyExceptionType gracefully handled:
 message="Smoked too many Cohibas!"
MyExceptionType gracefully handled:
 message="Smoked too many Cohibas!"
MyExceptionType gracefully handled:
 message="Smoked too many Cohibas!"
MyExceptionType gracefully handled:
 message="Smoked too many Cohibas!"
MyExceptionType gracefully handled:
 message="Smoked too many Cohibas!"

Process finished with exit code 0