如何教SQLAlchemy从断开恢复？

根据http://docs.sqlalchemy.org/en/rel_0_9/core/pooling.html#disconnect-handling-pessimistic，如果连接池中的条目不再有效，可以检测SQLAlchemy重新连接。 我创建了以下测试用例来测试它：

import subprocess
from sqlalchemy import create_engine, event
from sqlalchemy import exc
from sqlalchemy.pool import Pool

@event.listens_for(Pool, "checkout")
def ping_connection(dbapi_connection, connection_record, connection_proxy):
    cursor = dbapi_connection.cursor()
    try:
        print "pinging server"
        cursor.execute("SELECT 1")
    except:
        print "raising disconnect error"
        raise exc.DisconnectionError()
    cursor.close()

engine = create_engine('postgresql://postgres@localhost/test')

connection = engine.connect()

subprocess.check_call(['psql', str(engine.url), '-c',
    "select pg_terminate_backend(pid) from pg_stat_activity " +
    "where pid <> pg_backend_pid() " +
    "and datname='%s';" % engine.url.database],
    stdout=subprocess.PIPE)

result = connection.execute("select 'OK'")
for row in result:
    print "Success!", " ".join(row)

但不是恢复，我收到此异常：

sqlalchemy.exc.OperationalError: (OperationalError) terminating connection due to administrator command
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.

由于“ping服务器”被打印在终端上，所以可以安全地断定附加了事件监听器。 如何教SQLAlchemy从断开恢复？

---

它看起来像只有当你第一次从池中获得连接时才会调用checkout方法（例如，你的connection = engine.connect()行）

如果你后来失去连接，你将不得不明确地替换它，所以你可以拿一个新的连接，然后重试你的sql：

try:
    result = connection.execute("select 'OK'")
except sqlalchemy.exc.OperationalError:  # may need more exceptions here
    connection = engine.connect()  # grab a new connection
    result = connection.execute("select 'OK'")  # and retry

在sql的每一处都会遇到一些麻烦，所以你可以使用类似下面的方法来包装数据库查询：

def db_execute(conn, query):
    try:
        result = conn.execute(query)
    except sqlalchemy.exc.OperationalError:  # may need more exceptions here (or trap all)
        conn = engine.connect()  # replace your connection
        result = conn.execute(query)  # and retry
    return result

下列：

result = db_execute(connection, "select 'OK'")

现在应该成功。

另一种选择是也要监听invalidate方法，并在那时采取一些行动来取代你的连接。

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