AppEngine/Python: Why isn't the exception caught?

Question

I'm trying to write a Google-Appengine app that will fail nicely when datastore writes are disabled

Currently my main() looks like this:

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

---

If I set main to:

 def main(): run_wsgi_app(application) 

My app displays a traceback when the exception is raised.

---

If I set main to:

 def main(): run_wsgi_app(NoWrite) 

It will properly show my error message (although for every request).

---

Getting back to my modified version of main, this one:

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')

Instead of getting my error message, I still get a traceback that looks like this:

 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 

So, my question is, why isn't the exception caught?

Edit:

This function is from 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

the main function only register this application. Therefore, the exception will not raise in the main function. Therefore the try ... catch statement won't work.

The way to handle this exception is defining a new RequestHandler. Then, all requests which want to have this feature should inherent from the new RequestHandler.

for example:

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

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