响应Python中全局对象的异常

Question

我是Python新手，我偶然发现了一个异常处理问题。我正在编写一个简单的龙卷风+ momoko应用程序。总之，我有一个全局(?)对象是在主函数中创建的，该对象属于QueryExecutor类。它是一个使用momoko处理SQL查询执行的简单类。

class QueryExecutor:
    def __init__(self, database):
        self.db = database

    @gen.engine
    def _run(self, query):
        self.db.execute(query, callback = (yield gen.Callback('q')))
        try:
            cursor = yield momoko.WaitOp('q')
        except Exception as error:
            print(str(error))

    def save(self, tablename, data):
        fields = "(" + ", ".join(map(str, list(data.keys()))) + ")"
        values = "(" + "\'" + '\', \''.join(map(str, list(data.values()))) + "\'" + ")"
        query = "INSERT INTO " + tablename + " " + fields + " VALUES " + values + ";"
        self._run(query)

我想要实现的是在请求处理程序中使用这个类的对象，并在某种程度上能够判断何时发生了异常：

class RegistrationHandler(tornado.web.RequestHandler):
    def get(self):
        self.render("templates/register.html")

    def post(self):
        #...handle post arguments, retrieve user data, check if correct etc., do stuff...
        #...if everything ok:
        queryExec.save("users", userdata)
        #what to do if save threw an exception? 


application.db = momoko.Pool(dsn=dbsetup.dsn, size=1)


if __name__ == "__main__":
    queryExec = dbsetup.QueryExecutor(database=application.db)
    application.listen(8888)
    tornado.ioloop.IOLoop.instance().start()

当查询失败时，queryExec.save()会抛出一个异常，如果发生这种情况，我想在请求处理程序函数中知道。在try和not块中嵌入queryExec.save()不起作用。显然，我可能可以传递额外的参数(引用？)对于queryExec.save()，或者向QueryExecutor类本身添加某种状态参数，但是我想知道是否有更好的方法来解决这个问题？

编辑:修改后：

class TestEx(Exception): pass

以及：

@gen.engine
def _run(self, query):
    self.db.execute(query, callback = (yield gen.Callback('q')))
    try:
        cursor = yield momoko.WaitOp('q')
    except Exception as error:
        print(str(error))
        raise TestEx("test exception")

以及：

try:
    queryExec.save("users", userdata)
except dbsetup.TestEx as ex:
    print("exception caught in caller function")
    self.redirect("templates/login.html")

我进了控制台：

/usr/bin/python3.3 /home/tybur/PycharmProjects/tornadochat/main.py
duplicate key value violates unique constraint "unique_names"
DETAIL:  Key (name)=(testuser) already exists.

ERROR:tornado.application:Exception in callback None
Traceback (most recent call last):
  File "/home/tybur/PycharmProjects/tornadochat/dbsetup.py", line 46, in _run
    cursor = yield momoko.WaitOp('q')
  File "/usr/local/lib/python3.3/dist-packages/tornado/gen.py", line 520, in run
    next = self.yield_point.get_result()
  File "/usr/local/lib/python3.3/dist-packages/momoko/utils.py", line 59, in get_result
    raise error
  File "/usr/local/lib/python3.3/dist-packages/momoko/connection.py", line 244, in io_callback
    state = self.connection.poll()
psycopg2.IntegrityError: duplicate key value violates unique constraint "unique_names"
DETAIL:  Key (name)=(testuser) already exists.


During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.3/dist-packages/tornado/ioloop.py", line 688, in start
    self._handlers[fd](fd, events)
  File "/usr/local/lib/python3.3/dist-packages/tornado/stack_context.py", line 331, in wrapped
    raise_exc_info(exc)
  File "<string>", line 3, in raise_exc_info
  File "/usr/local/lib/python3.3/dist-packages/tornado/stack_context.py", line 302, in wrapped
    ret = fn(*args, **kwargs)
  File "/usr/local/lib/python3.3/dist-packages/momoko/connection.py", line 248, in io_callback
    self.callback(error)
  File "/usr/local/lib/python3.3/dist-packages/tornado/stack_context.py", line 331, in wrapped
    raise_exc_info(exc)
  File "<string>", line 3, in raise_exc_info
  File "/usr/local/lib/python3.3/dist-packages/tornado/stack_context.py", line 302, in wrapped
    ret = fn(*args, **kwargs)
  File "/usr/local/lib/python3.3/dist-packages/tornado/gen.py", line 574, in inner
    self.set_result(key, result)
  File "/usr/local/lib/python3.3/dist-packages/tornado/gen.py", line 500, in set_result
    self.run()
  File "/usr/local/lib/python3.3/dist-packages/tornado/gen.py", line 529, in run
    yielded = self.gen.throw(*exc_info)
  File "/home/tybur/PycharmProjects/tornadochat/dbsetup.py", line 49, in _run
    raise TestEx("test exception")
dbsetup.TestEx: test exception

Answer 1

save()不会引发异常；它会启动对_run的调用，但不会等待它，因此除了记录异常之外，没有什么地方可以让异常发生。要解决这个问题，您应该遵循三条规则：

• 除非您有特定的理由使用@gen.engine，否则应该使用@gen.coroutine，这使事情更像正常的函数。
• 任何调用协同线的方法都必须是协同线。这个规则有异常，但它们很微妙，除非您对异步编程有了更好的处理，否则您应该遵循它。因为只有coroutine可以调用coroutine，所以只能在特定位置启动链--通常在龙卷风文档中被描述为“可能返回未来”的方法(包括RequestHandler get/post/etc方法)。
• 对协同线的调用通常应该以“收益率”作为前缀，以等待其结果。如果在调用coroutine时不使用“Future”，则应该保存它的返回值( Future，它是其实际结果的占位符)，并在以后生成它(您可能希望这样做，并行启动几个coroutine，并同时等待它们)。

因此，在本例中，创建save()、_run()和post()协同，并在调用_run()和save()时使用_run()关键字。