python TorMySQL 異常處理不顯示traceback的坑

背景：

最近找到一個異步數據庫驅動，TorMySQL，ab測試後效果是其他幾個異步驅動最好的，遂用之。

因爲儘可能業務邏輯層和數據訪問層分離，並且考慮到單一負責原則，把每一個數據庫的操作簡稱爲service，如user_service，work_service之類的。

很多情況下，每一個邏輯業務都是要N個service合作才完成的。

所以就不在單個service中try except，而是在業務邏輯層中try except。

一是：

少寫很多代碼，不然每一個service都要try except之後，logging.error(ex, exc_info=1)打印traceback，然後再raise ex，多麻煩...

二是：

考慮到很多情況下有用到事務，只能在業務邏輯層捕獲exception，捕獲到就conn.rollback()，沒捕獲到就conn.commit()。

然而，我上面這種設計竟然沒捕獲到traceback，簡直匪夷所思。

好了，背景交代完畢。

版本：

python 2.7
tormysql==0.2.9
tornado==4.2

測試1：

import logging

from tornado.ioloop import IOLoop
from tornado import gen
import tormysql

pool = tormysql.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 = "123qwe",
    db = "chanzai_dev",
    charset = "utf8"
)

@gen.coroutine
def aa(conn):
        sql = "SELECT * FROM UserDevice where user_id = %s" % 100002
        with conn.cursor() as cursor:
            yield cursor.execute(sql)
            cursor.fetchone()
            # 下面是故意出錯代碼
            a = '10'
            b = a / 10
            print b
@gen.coroutine
def bb(conn):
    yield aa(conn)

@gen.coroutine
def cc():
    with (yield pool.Connection()) as conn:
        try:
            yield bb(conn)
        except KeyError:
            pass
        except Exception, ex:
            yield conn.rollback()
            logging.error(ex, exc_info=1)

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

測試結果：

G:\projects\myApp\tmp>a.py
ERROR:root:unsupported operand type(s) for /: 'str' and 'int'
None

測試2：

import logging

from tornado.ioloop import IOLoop
from tornado import gen
import tormysql

pool = tormysql.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 = "123qwe",
    db = "chanzai_dev",
    charset = "utf8"
)

@gen.coroutine
def aa(conn):
        sql = "SELECT * FROM UserDevice where user_id = %s" % 100002
        with conn.cursor() as cursor:
            yield cursor.execute(sql)
            cursor.fetchone()
            # 下面是故意出錯代碼
            a = '10'
            b = a / 10
            print b
@gen.coroutine
def bb(conn):
    yield aa(conn)

@gen.coroutine
def cc():
    with (yield pool.Connection()) as conn:
        try:
            yield bb(conn)
        except KeyError:
            pass
        except Exception, ex:
            logging.error(ex, exc_info=1)
            yield conn.rollback()

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

測試結果：

G:\projects\myApp\tmp>a.py
ERROR:root:unsupported operand type(s) for /: 'str' and 'int'
Traceback (most recent call last):
  File "G:\projects\myApp\tmp\a.py", line 40, in cc
    yield bb(conn)
  File "G:\Python27\lib\site-packages\tornado\gen.py", line 1015, in run
    value = future.result()
  File "G:\Python27\lib\site-packages\tornado\concurrent.py", line 237, in result
    raise_exc_info(self._exc_info)
  File "G:\Python27\lib\site-packages\tornado\gen.py", line 1021, in run
    yielded = self.gen.throw(*exc_info)
  File "G:\projects\myApp\tmp\a.py", line 34, in bb
    yield aa(conn)
  File "G:\Python27\lib\site-packages\tornado\gen.py", line 1015, in run
    value = future.result()
  File "G:\Python27\lib\site-packages\tornado\concurrent.py", line 237, in result
    raise_exc_info(self._exc_info)
  File "G:\Python27\lib\site-packages\tornado\gen.py", line 1024, in run
    yielded = self.gen.send(value)
  File "G:\projects\myApp\tmp\a.py", line 30, in aa
    b = a / 10
TypeError: unsupported operand type(s) for /: 'str' and 'int'

G:\projects\myApp\tmp>

觀察結果：

根據兩個測試，其實只改了一個地方，就是：

        except Exception, ex:
            logging.error(ex, exc_info=1)
            yield conn.rollback()

和

        except Exception, ex:
            yield conn.rollback()
            logging.error(ex, exc_info=1)

前者才能得到想要的結果，爲什麼有後者這種寫法呢，是因爲，想着對於數據庫的操作出錯了還是儘早rollback爲好，沒想到就踩了坑。

原因分析：

看了下pymysql的connections.py源碼（因爲TorMySQL是對pymysql的封裝），發現確實有很多try except，我都不用具體往下看了，因爲很大可能性是我們的ex被pymysql給捕獲到了，但是raise出來的是pymysql處理過的ex，所以看不到traceback...吧

解決方案就是先logging.error(ex, exc_info=1) 再 yield conn.rollback()