django.db.__init__.py 
#對象：
connections = ConnectionHandler()
connection = DefaultConnectionProxy()
# 函數(shù)
# 重置查詢記錄緩存
def reset_queries(**kwargs):
 pass
# 關(guān)閉不可用或超時(如果有設(shè)置 CONN_MAX_AGE)連接
def close_old_connections(**kwargs):
 pass
# 信號
# 在請求開始或完成時自動調(diào)用相應(yīng)處理函數(shù)
signals.request_started.connect(reset_queries)
signals.request_started.connect(close_old_connections)
signals.request_finished.connect(close_old_connections)

def __init__()：
 self._connections = local()

# 連接包裝類里的連接是根據(jù)配置情況使用相應(yīng)的連接
def __getitem__(self, alias):
 '''略'''
 db = self.databases[alias]
 backend = load_backend(db['ENGINE'])
 conn = backend.DatabaseWrapper(db, alias)
 setattr(self._connections, alias, conn)

# 返回所管理的數(shù)據(jù)庫連接
# 管理方式：分?jǐn)?shù)據(jù)庫，線程管理連接 
def all(self):
 return [self[alias] for alias in self]

# 關(guān)閉所有數(shù)據(jù)庫連接
def close_all(self):
 for alias in self:
  try:
   connection = getattr(self._connections, alias)
  except AttributeError:
   continue
  connection.close()

connection是DefaultConnectionProxy類的實例，實際是DatabaseWrapper的實例 
（使用了pymysql庫：import pymysql; pymysql.install_as_MySQLdb） 
DefaultConnectionProxy–>DatabaseWrapper–>pymysql.connections.Connection(根據(jù)connections的處理調(diào)用相應(yīng)的數(shù)據(jù)庫連接包) 
connection有幾個關(guān)鍵方法和屬性

connection.connection = '被包裝的pymysql.connections.Connection實例`
connection.close_at = None if max_age is None else time.time() + max_age # 設(shè)置的連接關(guān)閉時間

connection.connect()# 獲取連接
connection.cursor() # 獲取游標(biāo)
connection.close()# 關(guān)閉連接

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!"