Psycopg2：退出包含 try/except 的 'with' 语句时连接未提交

Question

首先，我想声明我很清楚 psycopg 打开一个事务，需要提交才能进行数据库更新。

但是这里引用了关于提交的文档：

commit()

Commit any pending transaction to the database.

By default, Psycopg opens a transaction before executing the first command: if commit() is not called, the effect of any data manipulation will be lost.

The connection can be also set in “autocommit” mode: no transaction is automatically open, commands have immediate effect. See Transactions control for details.

Changed in version 2.5: if the connection is used in a with statement, the method is automatically called if no exception is raised in the with block.

回滚也类似：

rollback()

Roll back to the start of any pending transaction. Closing a connection without committing the changes first will cause an implicit rollback to be performed.

Changed in version 2.5: if the connection is used in a with statement, the method is automatically called if an exception is raised in the with block.

然而这似乎对我不起作用。也许我很挑剔，并且应该始终养成显式提交的习惯（但是如果我忘记它会发生回滚，那就太好了）。我想知道是否有人遇到了同样的问题，或者我是否错误地使用了我的 with 语句。

我的代码如下所示：

import psycopg2
from contextlib import closing

with closing(psycopg2.connect(
                               #conn params
                              ) as conn, \
     closing(conn.cursor()) as cur:

     try:
         sql = #query
         cur.execute(sql)

         output = cur.statusmessage
         print output

     except Exception, e :
         print "Error message : {}".format(e)
         raise

这行不通。查询本身没有问题，因为我得到：更新 842518 正如预期的那样输出。但是，由于我的数据库未更新，因此显然未提交来自连接的更改。如果我添加

conn.commit()

在 try 语句中它有效，但重点是避免这样做。

我的 psycopg2 版本是 2.6。

Link to doc

Answer 1

在关闭时包装连接和游标对象很可能会扰乱它们的上下文处理； they are context managers them selves。 closing 包装器只会在退出时调用它们的 close() 方法。

在 with 块中使用游标时无需手动调用 close()。这同样不适用于连接；只有事务在退出 with 块时结束。这是设计使然，因此您可以在多个 with 块中使用相同的连接：

Note that, unlike file objects or other resources, exiting the connection’s with block doesn’t close the connection but only the transaction associated with it: a connection can be used in more than a with statement and each with block is effectively wrapped in a separate transaction

例如，您可以将 closing 中的连接包装在最外层的 with-block 中，然后在内部 with-block 中使用相同的连接进行事务处理等：

# This'll handle closing the connection
with closing(psycopg2.connect(...)) as conn:
    # This'll handle the transaction and closing the cursor
    with conn, conn.cursor() as cur:
        ...

Psycopg2：退出包含 try/except 的 'with' 语句时连接未提交

Psycopg2: connection not committing when exiting 'with' statement containing a try/except

python

psycopg2

python-2.7