Question

SQLAlchemy CORE doc for transactions建议使用with上下文管理器，如下所示：

# runs a transaction
with engine.begin() as connection:
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), col1=7, col2='this is some data')

或

with connection.begin() as trans:
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), col1=7, col2='this is some data')

无论哪种方式，我如何知道交易是否已被执行和提交，或者是否已被回滚？

Answer 1

由于在发生异常时执行回滚，如果执行with之后的代码，您可以放心，该事务尚未回滚。

如果您还需要测试，那么您可以使用通常的python结构，例如：

try:
    with connection.begin() as trans:
        r1 = connection.execute(table1.select())
        connection.execute(table1.insert(), col1=7, col2='this is some data')
except:
    # The transaction has been rolled back, you can log or otherwise
    # react to that fact
    ...
    # Finally, propagate the exception to the caller. If you choose not
    # to propagate, then don't use `except:`, but handle a narrower
    # class of exceptions.
    raise

Answer 2

如果它没有提高，它就会提交。如果您查看文档，您会注意到with语句或多或少等同于：

connection = engine.connect()
trans = connection.begin()
try:
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), col1=7, col2='this is some data')
    trans.commit()
except:
    trans.rollback()
    raise

关于注释：with语句不一定是try /的替代，除非您需要异常处理 - 例如当您想知道事务是否回滚时。

如果您必须执行额外的清理或例如在事务回滚时进行日志记录，您仍然需要在try / except中包装with语句，但是您可以确定该事务已被处理在控制之前从由with语句控制的块传递：

try:
    with ...:
        ...

except ...:
    # rolled back

else:
    # committed

您也可以选择重新加载错误，以便其他部分也可以处理清理。当然，例如日志记录也可以由另一个上下文管理器处理：

from contextlib import contextmanager

@contextmanager
def logger(log, error_msg="Oh woe!"):
    try:
        yield

    except:
        log.exception(error_msg)
        raise

...

with logger(log), connection.begin():
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), col1=7, col2='this is some data')

在这种情况下，正如您在评论中指出的那样，PEP 343的精神使用with语句消除或隐藏了try / except：

这个PEP添加了一个新的声明＆＃34;＆＃34;使用Python语言可以分解try / finally语句的标准用法。

在SQLAlchemy Core中使用`with`上下文管理器如何知道事务是否回滚？

2 个答案: