python - 应用引擎/ python : Why isn't the exception caught?

Question

我正在尝试编写一个 Google-Appengine 应用程序，当数据存储写入被禁用时，它会很好地失败

目前我的 main() 看起来像这样:

def main():
    make_datastore_readonly()
    try:
        run_wsgi_app(application)
    except CapabilityDisabledError:
        run_wsgi_app(NoWrite)

---

如果我将 main 设置为:

def main():
    run_wsgi_app(application)

我的应用程序在引发异常时显示回溯。

---

如果我将 main 设置为:

def main():
    run_wsgi_app(NoWrite)

它将正确显示我的错误消息(尽管对于每个请求)。

---

回到我修改后的 main 版本，这个:

def main():
    make_datastore_readonly()
    try:
        run_wsgi_app(application)
    except CapabilityDisabledError:
        run_wsgi_app(NoWrite)

我没有收到错误消息，而是得到如下所示的回溯:

Traceback (most recent call last):
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/webapp/_webapp25.py", line 703, in __call__
    handler.post(*groups)
  File "/Users/kevin/Sche/main.py", line 232, in post
    me.put();
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/db/__init__.py", line 1074, in put
    return datastore.Put(self._entity, **kwargs)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/api/datastore.py", line 579, in Put
    return PutAsync(entities, **kwargs).get_result()
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/api/datastore.py", line 556, in PutAsync
    return _GetConnection().async_put(config, entities, local_extra_hook)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/datastore/datastore_rpc.py", line 1553, in async_put
    return make_put_call(base_req, pbs, extra_hook)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/datastore/datastore_rpc.py", line 1543, in make_put_call
    self.__put_hook, user_data)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/datastore/datastore_rpc.py", line 1188, in make_rpc_call
    rpc.make_call(method, request, response, get_result_hook, user_data)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/api/apiproxy_stub_map.py", line 519, in make_call
    self.__service, method, request, response, self.__rpc)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/api/apiproxy_stub_map.py", line 207, in Call
    function(service, call, request, response)
  File "/Users/kevin/Sche/main.py", line 18, in hook
    raise CapabilityDisabledError('Datastore is in read-only mode')
CapabilityDisabledError: Datastore is in read-only mode

所以，我的问题是，为什么没有捕获异常？

编辑:

这个函数来自this StackOverflow answer

def make_datastore_readonly():
  """Throw ReadOnlyError on put and delete operations."""
  def hook(service, call, request, response):
    assert(service == 'datastore_v3')
    if call in ('Put', 'Delete'):
      raise CapabilityDisabledError('Datastore is in read-only mode') //Line 18
  apiproxy_stub_map.apiproxy.GetPreCallHooks().Push('readonly_datastore', hook, 'datastore_v3')

Answer 1

main 函数只注册这个应用程序。因此，不会在主函数中引发异常。因此 try ... catch 语句将不起作用。

处理这个异常的方法是定义一个新的RequestHandler。然后，所有希望具有此功能的请求都应该从新的 RequestHandler 中继承。

例如:

Class MyRequestHandler(RequestHandler):
    def get(self):
        try:
            self.get_handler()
        except CapabilityDisabledError:
            pass

class MyRequest(MyRequestHandler):
    def get_handler(self):
        # ....
        pass