Python2.7.6:如何正确地清理类？

Question

在下面的可执行代码中，您可以看到一个SessionScope()-class。在main()-function中，用户可以登录到他的MySQL数据库服务器。我们在班上看一看。有两种神奇的方法-- (__enter__, __exit__)，它允许我使用with-statement轻松地使用对象。在这个语句中，您还可以看到程序使用会话。当__exit__()-method正在调用时，会话就会关闭。但我们知道这将使连接返回到引擎的连接池。这意味着，它不会直接关闭连接，因为连接是池的。到目前一切尚好。在GUI方面，用户可以选择注销。那么，让我们想象一下:经过很长时间的数据库工作之后，用户希望连接实际上被关闭，但他不希望程序自己关闭。稍后，用户可能会再次登录并继续工作。在此之前，程序仍在运行，没有连接到数据库。用户不再需要连接了。

这意味着对于python，我们不再需要SessionScope()-class了。在我的例子中，我们可以使用del session_scope删除/清理这个类，我的想法是重新实现__del__()-method。在此方法中，我希望关闭连接池的所有连接。如果这个类被清除/删除，那么所有的连接都应该断开连接，这就是为什么我在log_out()-function中使用-function的原因。

这样做对吗？

塔，你的索菲斯

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.exc import SQLAlchemyError

class SessionScope(object):
    def __init__(self, dbms, dbdriver, dbuser, dbuser_pwd, db_server_host, dbport, db_name):

        self.dbms = dbms
        self.dbdriver = dbdriver
        self.dbuser = dbuser
        self.dbuser_pwd = dbuser_pwd
        self.db_server_host = db_server_host
        self.dbport = dbport
        self.db_name = db_name

        url = '{}+{}://{}:{}@{}:{}/{}'.format(
           self.dbms, self.dbdriver, self.dbuser, self.dbuser_pwd, self.db_server_host, self.dbport, self.db_name)

        self.engine = create_engine(url, encoding='utf8', echo=True)

        # store a sessionmaker for this db connection object
        self._Session = sessionmaker(bind=self.engine)
        self.session = None

    def __enter__(self):
        self.session = self._Session()
        return self._Session()

    def __exit__(self, exception, exc_value, traceback):

        try:
            if exception:
                self.session.rollback()
            else:
                self.session.commit()
        finally:

            self.session.close()
            self.session = None

    def __del__(self):
        self.engine.dispose()

def log_out(session_scope):
    del session_scope

def main():
    dbm_system = raw_input("Which DBMS? (type for e.g. mysql): ")
    dbm_driver = raw_input("Which db-driver? (type for e.g. pymysql): ")
    db_host = raw_input("Server-Host: ")
    db_user = raw_input("Database-user: ")
    db_passwd = raw_input("User-Password: ")
    db_name = raw_input("Database Name: ")
    db_port = raw_input("Port: ")

    try:
        session_scope = SessionScope(dbm_system, dbm_driver, db_user, \
                    db_passwd, db_host, db_port, db_name)

        with session_scope as session:
            # Its just for testing.
            print session.execute("SELECT VERSION();")

        log_out(session_scope)

    except SQLAlchemyError as err:
        print "ERROR", err[0]


if __name__ == '__main__':
    main()

编辑#1:

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.exc import SQLAlchemyError

class SessionScope(object):
    def __init__(self, engine):

        self.engine = engine

        # store a sessionmaker for this db connection object
        self._Session = sessionmaker(bind=self.engine)
        self.session = None

    def __enter__(self):
        self.session = self._Session()
        return self._Session()

    def __exit__(self, exception, exc_value, traceback):

        try:
            if exception:
                self.session.rollback()
            else:
                self.session.commit()
        finally:

            self.session.close()
            self.session = None

class Engine(object):
    def __init__(self, dbms, dbdriver, dbuser, dbuser_pwd, db_server_host, dbport, db_name):

        self.dbms = dbms
        self.dbdriver = dbdriver
        self.dbuser = dbuser
        self.dbuser_pwd = dbuser_pwd
        self.db_server_host = db_server_host
        self.dbport = dbport
        self.db_name = db_name

        url = '{}+{}://{}:{}@{}:{}/{}'.format(
           self.dbms, self.dbdriver, self.dbuser, self.dbuser_pwd, self.db_server_host, self.dbport, self.db_name)

        self._Engine = create_engine(url, encoding='utf8', echo=True)

    def __enter__(self):
        return self._Engine

    def __exit__(self, exception, exc_value, traceback):
        '''
            Make sure the dbconnection gets closed
        '''
        self._Engine.dispose()

logged_in = True

def main():

    dbm_system = raw_input("Which DBMS? (type for e.g. mysql): ")
    dbm_driver = raw_input("Which db-driver? (type for e.g. pymysql): ")
    db_host = raw_input("Server-Host: ")
    db_user = raw_input("Database-user: ")
    db_passwd = raw_input("User-Password: ")
    db_name = raw_input("Database Name: ")
    db_port = raw_input("Port: ")

    try:
        with Engine(dbm_system, dbm_driver, db_user, \
                    db_passwd, db_host, db_port, db_name) as engine:

            while logged_in:
                with SessionScope(engine) as session:
                    # Its just for testing.
                    print session.execute("SELECT VERSION();")

    except SQLAlchemyError as err:
        print "ERROR", err[0]


if __name__ == '__main__':
    main()

Answer 1

__del__方法不是那样工作的。它不是在用户执行del some_instance时调用的，而是当解释器的垃圾收集器看到对象没有活动引用时调用的。您的log_out方法什么也不做，因为它正在删除的引用是为将会话作为参数传递给它而创建的额外引用(外部引用仍然保留)。

我怀疑您确实希望有两个不同的类，它们都支持上下文管理器协议。这允许您有两个嵌套的with语句，一个持续到整个登录，每次数据库会话只持续一个。就像这样：

with Engine() as engine:
    while logged_in:
        with Session(engine) as session:
            do_stuff()

您可能需要在外部with周围再循环一次，这样程序就不会在注销后退出。