django.db.__init__.py 
#对象：
connections = ConnectionHandler()
connection = DefaultConnectionProxy()
# 函数
# 重置查询记录缓存
def reset_queries(**kwargs):
 pass
# 关闭不可用或超时(如果有设置 CONN_MAX_AGE)连接
def close_old_connections(**kwargs):
 pass
# 信号
# 在请求开始或完成时自动调用相应处理函数
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()

# 连接包装类里的连接是根据配置情况使用相应的连接
def __getitem__(self, alias):
 '''略'''
 db = self.databases[alias]
 backend = load_backend(db['ENGINE'])
 conn = backend.DatabaseWrapper(db, alias)
 setattr(self._connections, alias, conn)

# 返回所管理的数据库连接
# 管理方式：分数据库，线程管理连接 
def all(self):
 return [self[alias] for alias in self]

# 关闭所有数据库连接
def close_all(self):
 for alias in self:
  try:
   connection = getattr(self._connections, alias)
  except AttributeError:
   continue
  connection.close()

connection是DefaultConnectionProxy类的实例，实际是DatabaseWrapper的实例 
（使用了pymysql库：import pymysql; pymysql.install_as_MySQLdb） 
DefaultConnectionProxy–>DatabaseWrapper–>pymysql.connections.Connection(根据connections的处理调用相应的数据库连接包) 
connection有几个关键方法和属性

connection.connection = '被包装的pymysql.connections.Connection实例`
connection.close_at = None if max_age is None else time.time() + max_age # 设置的连接关闭时间

connection.connect()# 获取连接
connection.cursor() # 获取游标
connection.close()# 关闭连接

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!"