Python 2.7.6：如何正確清理類？

Question

在以下可執行代碼中，您看到一個SessionScope（）類。在main（）中，用戶可以登錄到他的MySQL數據庫服務器。我們在課堂上看看。有兩種神奇的方法(__enter__, __exit__), ，它允許我用with -statement輕鬆使用對象。在此聲明中，您還會看到該程序使用會話。當__exit__（） - 方法調用 時，會話關閉。但是我們知道會將連接返回到Engine的連接池。這意味着，它不會直接關閉連接，因爲連接是共享的，所以 。到現在爲止還挺好。在GUI一側，用戶可以選擇註銷。那麼，讓我們想象一下：經過非常非常非常長的數據庫工作，用戶希望連接真正關閉，但他不希望程序自行關閉。稍後，用戶可能會再次登錄並繼續工作。在此之前，該程序仍在運行，但未連接到數據庫 。用戶不再需要連接。

這意味着對於python，我們不再需要SessionScope（）類。在我的情況下，我們可以用del session_scope刪除/清理這個類。我的想法是重新實現__del__（） - 方法。在這種方法中，我想 關閉連接池的所有連接。如果這個類被清除/刪除，所有的連接都應該斷開，這就是爲什麼我使用log_out（）函數中的del的原因。

這是正確的方法嗎？

TA，你Sophus

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() 

編輯＃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

的__del__方法不喜歡的工作。當用戶執行del some_instance時不會調用它，但是當解釋器的垃圾回收器看到沒有對該對象的實時引用時。你的log_out方法什麼都不做，因爲它所刪除的引用是爲了將會話作爲參數傳遞給它而創建的（外部引用仍然存在）。

我懷疑你真的想擁有兩個不同的類，它們都支持上下文管理器協議。這可以讓你有兩個嵌套的with語句，它們貫穿整個登錄並且只持續每個數據庫會話的長度。事情是這樣的：

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

您可能需要外with，使您註銷後，該程序不會退出身邊另一個循環。