TorMySQL 封装的 async with 语句抛出了 AssertionError 异常

说明

我的数据库和表都是没有问题的，不会有数据库不存在或者表结构错误的情况
我用的是tornado 4.4.2，Tornado的文档中说到 Tornado 4.3后就支持Python 3中的async/await 关键字了。

正文

我这两天了解了一下 Python3 中的 async/await 语句，然后照猫画虎，对 TorMySQL 封装了一个 async with语句的上下文管理器：

代码如下：

#!/usr/bin/env python3
# -*- coding: utf-8 -*-


from tornado.ioloop import IOLoop
import tormysql

class get_cursor:
    def __init__(self, pool):
    	""" get_cursor 上下文管理器
        
        针对`async with`语句封装的上下文管理器
        """
        self.pool = pool

    async def __aenter__(self):
        self.tx = await self.pool.begin()
        return self.tx

    async def __aexit__(self, exc_type, exc, tb):
        if tb is None:
            self.tx.commit()
        else:
            self.tx.rollback()

async def insert():
    pool = tormysql.helpers.ConnectionPool(
        max_connections = 20, #max open connections
        idle_seconds = 7200, #conntion idle timeout time, 0 is not timeout
        wait_connection_timeout = 3, #wait connection timeout
        host = "127.0.0.1",
        user = "root",
        passwd = "TEST",
        db = "test",
        charset = "utf8"
    )
    async with get_cursor(pool) as cursor:
        cursor.execute("INSERT INTO test(id) VALUES(1)")

ioloop = IOLoop.instance()
ioloop.run_sync(insert)

我执行了上述代码以后，抛出了如下的错误：

➜ /home/yundongx/tutorial/async [async]$ python3 async.py
ERROR:tornado.application:Future <tornado.concurrent.Future object at 0x7f1983708470> exception was never retrieved: Traceback (most recent call last):
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/tornado/gen.py", line 1021, in run
    yielded = self.gen.throw(*exc_info)
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/tormysql/helpers.py", line 44, in commit
    yield self._connection.commit()
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/tornado/gen.py", line 1015, in run
    value = future.result()
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/tornado/concurrent.py", line 237, in result
    raise_exc_info(self._exc_info)
  File "<string>", line 3, in raise_exc_info
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/tormysql/util.py", line 14, in finish
    result = fun(*args, **kwargs)
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/pymysql/connections.py", line 767, in commit
    self._read_ok_packet()
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/pymysql/connections.py", line 746, in _read_ok_packet
    pkt = self._read_packet()
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/pymysql/connections.py", line 961, in _read_packet
    packet_header = self._read_bytes(4)
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/tormysql/connections.py", line 308, in _read_bytes
    future = self._rfile.read_bytes(num_bytes)
  File "/home/yundongx/.virtualenvs/async/lib/python3.5/site-packages/tormysql/connections.py", line 97, in read
    assert self._read_future is None, "Already reading"
AssertionError: Already reading
Exception ignored in: <generator object execute at 0x7f19836e2e60>
RuntimeError: generator ignored GeneratorExit
WARNING:root:Transaction has not committed or rollbacked <tormysql.connections.Connection object at 0x7f19837080b8> {'user': b'root', 'port': 3306, 'host': '127.0.0.1', 'database': b'test'}.

然后我去数据库中查询这个表，发现数据已经插入成功了。

请问一下这个异常是什么意思啊（ PyMySQL 显示说Already reading是什么意思啊）？