django.db.__init__.py 
#¶ÔÏó£º
connections = ConnectionHandler()
connection = DefaultConnectionProxy()
# º¯Êý
# ÖØÖòéѯ¼Ç¼»º´æ
def reset_queries(**kwargs):
 pass
# ¹Ø±Õ²»¿ÉÓûò³¬Ê±(Èç¹ûÓÐÉèÖà CONN_MAX_AGE)Á¬½Ó
def close_old_connections(**kwargs):
 pass
# ÐźÅ
# ÔÚÇëÇó¿ªÊ¼»òÍê³Éʱ×Ô¶¯µ÷ÓÃÏàÓ¦´¦Àíº¯Êý
signals.request_started.connect(reset_queries)
signals.request_started.connect(close_old_connections)
signals.request_finished.connect(close_old_connections)

def __init__()£º
 self._connections = local()

# Á¬½Ó°ü×°ÀàÀïµÄÁ¬½ÓÊǸù¾ÝÅäÖÃÇé¿öʹÓÃÏàÓ¦µÄÁ¬½Ó
def __getitem__(self, alias):
 '''ÂÔ'''
 db = self.databases[alias]
 backend = load_backend(db['ENGINE'])
 conn = backend.DatabaseWrapper(db, alias)
 setattr(self._connections, alias, conn)

# ·µ»ØËù¹ÜÀíµÄÊý¾Ý¿âÁ¬½Ó
# ¹ÜÀí·½Ê½£º·ÖÊý¾Ý¿â£¬Ï̹߳ÜÀíÁ¬½Ó 
def all(self):
 return [self[alias] for alias in self]

# ¹Ø±ÕËùÓÐÊý¾Ý¿âÁ¬½Ó
def close_all(self):
 for alias in self:
  try:
   connection = getattr(self._connections, alias)
  except AttributeError:
   continue
  connection.close()

connectionÊÇDefaultConnectionProxyÀàµÄʵÀý£¬Êµ¼ÊÊÇDatabaseWrapperµÄʵÀý 
£¨Ê¹ÓÃÁËpymysql¿â£ºimport pymysql; pymysql.install_as_MySQLdb£© 
DefaultConnectionProxy¨C>DatabaseWrapper¨C>pymysql.connections.Connection(¸ù¾ÝconnectionsµÄ´¦Àíµ÷ÓÃÏàÓ¦µÄÊý¾Ý¿âÁ¬½Ó°ü) 
connectionÓм¸¸ö¹Ø¼ü·½·¨ºÍÊôÐÔ

connection.connection = '±»°ü×°µÄpymysql.connections.ConnectionʵÀý`
connection.close_at = None if max_age is None else time.time() + max_age # ÉèÖõÄÁ¬½Ó¹Ø±Õʱ¼ä

connection.connect()# »ñÈ¡Á¬½Ó
connection.cursor() # »ñÈ¡Óαê
connection.close()# ¹Ø±ÕÁ¬½Ó

max_age = 7 * 3600 

if not db.connection.connection or db.connection.close_at < time.time():
 db.connection.close()
 db.connection.connect()
 db.connection.close_at = time.time() + max_age
 print "A new conn creates !"
else:
 print "Still old conn!"