I have some Klein code that uses deferToThread for I/O. It looks something like this: @app.route('/', methods=['GET'] def index(request, *args, **kwargs): d = deferToThread(some_blocking_db_select_function) def serialize(db_object): return json.dumps({ 'id': db_object, }) d.addCallback(serialize) return d I have a test that executes this function, and the deferToThread returns a Deferred, but that deferred never fires its callback, and so when I use successResultOf expecting a success result, no result is found. When I use twistd to run the Klein application, everything works fine (the deferred fires and I get a json string in the body of the response). What's different about the trial environment that deferToThread might not fire its callback? Do I need to explicitly set up a thread pool in trial that I don't have to set up using twistd? Any help would be appreciated. -Piper