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Python学习 - 编写自己的ORM(2)
上一篇文章简单的实现了ORM(对象关系模型),这一篇文章主要实现简单的MySQL数据库操作。
想要操作数据库,首先要建立一个数据库连接。下面定义一个创建数据库连接的函数,得到一个连接叫做engine。
def create_engine(user,password,database,host=‘127.0.0.1‘,port=3306,**kw): import mysql.connector global engine if engine is not None: raise DBError(‘Engine is already initialized.‘) params = dict(user=user,password=password,database=database,host=host,port=port) defaults = dict(use_unicode=True,charset=‘utf8‘,collation=‘utf8_general_ci‘,autocommit=False) #print (‘%s %s %s %s %s‘) % (user,password,database,host,port) for k,v in defaults.iteritems(): params[k] = kw.pop(k,v) params.update(kw) params[‘buffered‘] = True engine = mysql.connector.connect(**params) cursor = engine.cursor()
有了连接就可以对数据库进行操作了。下面写了几个函数,可以对数据库进行查询和插入操作。
def _select(sql,first,*args): cursor = None sql = sql.replace(‘?‘,‘%s‘) global engine try: cursor = engine.cursor() cursor.execute(sql,args) if cursor.description: names = [x[0] for x in cursor.description] if first: values = cursor.fetchone() if not values: return None return Dict(names,values) return [Dict(names,x) for x in cursor.fetchall()] finally: if cursor: cursor.close()def select_one(sql,*args): return _select(sql,True,*args)def select(sql,*args): return _select(sql,False,*args)def _update(sql,*args): cursor = None global engine sql = sql.replace(‘?‘,‘%s‘) print sql try: cursor = engine.cursor() cursor.execute(sql,args) r = cursor.rowcount engine.commit() return r finally: if cursor: cursor.close()def insert(table,**kw): cols, args = zip(*kw.iteritems()) sql = ‘insert into %s (%s) values(%s)‘ % (table,‘,‘.join([‘%s‘ % col for col in cols]),‘,‘.join([‘?‘ for i in range(len(cols))])) print (‘sql %s args %s‘ % (sql, str(args))) return _update(sql,*args)
到这里,基本的数据库操作已经完成了。但是,根据廖雪峰的教程,这还远远不够。
- 如果要在一个数据库连接中实现多个操作,上面的代码效率很低,没次执行玩一条语句,就需要重新分配一个连接。
- 在一次事务中执行多条操作也是一样效率低下。
- 如果服务器为不同用户数据库请求都分配一个线程来建立连接,但是在进程中,连接是可供享使用的。这样问题就来了,导致数据库操作可能异常。
针对第三个问题,应该使每个连接是每个线程拥有的,其它线程不能访问,使用threading.local。首先定义一个类,来保存数据库的上下文:
class _DbCtx(threading.local): def __init__(self): self.connection = None self.transactions = 0 def is_init(self): return not self.connection is None def init(self): self.connection = engine # 创建数据库连接 self.transactions = 0 def cleanup(self): self.connection.cleanup() self.connection = None def cursor(self): return self.connection.cursor()
上面的代码有一个错误。因为Python的赋值语句只是将一个对象的引用传给一个变量,就如上面代码中 init函数中 self.connection = engine。表明self.connection和engine都指向一个数据库连接的对象。如果将self.connection给cleanup了,那么engine指向的对象也被cleanup了。下图是一个例子:
a是类foo实例的一个引用,执行b=a后,在执行b.clean(),此时应该只是b的v值被更改为0,但是执行a.v却发现v的值也变为0了。
下面是最后的代码,只是封装了最底层的数据库操作,代码也写的很涨,虽然是模仿廖雪峰的代码。
# -*- coding: utf-8 -*-import time, uuid, functools, threading, loggingclass Dict(dict): ‘‘‘ Simple dict but support access as x.y style. ‘‘‘ def __init__(self, names=(), values=(), **kw): super(Dict, self).__init__(**kw) for k, v in zip(names, values): self[k] = v def __getattr__(self, key): try: return self[key] except KeyError: raise AttributeError(r"‘Dict‘ object has no attribute ‘%s‘" % key) def __setattr__(self, key, value): self[key] = valueclass DBError(Exception): passclass MultiColumnsError(Exception): passengine = Noneclass _DbCtx(threading.local): def __init__(self): self.connection = None self.transactions = 0 def is_init(self): return not self.connection is None def init(self): self.connection = engine self.transactions = 0 def cleanup(self): self.connection = None def cursor(self): return self.connection.cursor()def create_engine(user,password,database,host=‘127.0.0.1‘,port=3306,**kw): import mysql.connector global engine if engine is not None: raise DBError(‘Engine is already initialized.‘) params = dict(user=user,password=password,database=database,host=host,port=port) defaults = dict(use_unicode=True,charset=‘utf8‘,collation=‘utf8_general_ci‘,autocommit=False) #print (‘%s %s %s %s %s‘) % (user,password,database,host,port) for k,v in defaults.iteritems(): params[k] = kw.pop(k,v) params.update(kw) params[‘buffered‘] = True engine = mysql.connector.connect(**params) print type(engine)_db_ctx = _DbCtx()class _ConnectionCtx(object): def __enter__(self): self.should_cleanuo = False if not _db_ctx.is_init(): cursor = engine.cursor() _db_ctx.init() self.should_cleanup = True return self def __exit__(self,exctype,excvalue,traceback): if self.should_cleanup: _db_ctx.cleanup()def with_connection(func): @functools.wraps(func) def _wrapper(*args,**kw): with _ConnectionCtx(): return func(*args, **kw) return _wrapperdef _select(sql,first,*args): cursor = None sql = sql.replace(‘?‘,‘%s‘) global _db_ctx try: cursor = _db_ctx.cursor() cursor.execute(sql,args) if cursor.description: names = [x[0] for x in cursor.description] if first: values = cursor.fetchone() if not values: return None return Dict(names,values) return [Dict(names,x) for x in cursor.fetchall()] finally: if cursor: cursor.close()@with_connectiondef select_one(sql,*args): return _select(sql,True,*args)@with_connectiondef select_int(sql,*args): d = _select(sql,True,*args) if len(d) != 1: raise MultoColumnsError(‘Except only one column.‘) return d.values()[0]@with_connectiondef select(sql,*args): global engine print type(engine) return _select(sql,False,*args)@with_connectiondef _update(sql,*args): cursor = None global _db_ctx sql = sql.replace(‘?‘,‘%s‘) print sql try: cursor = _db_ctx.cursor() cursor.execute(sql,args) r = cursor.rowcount engine.commit() return r finally: if cursor: cursor.close()def insert(table,**kw): cols, args = zip(*kw.iteritems()) sql = ‘insert into %s (%s) values(%s)‘ % (table,‘,‘.join([‘%s‘ % col for col in cols]),‘,‘.join([‘?‘ for i in range(len(cols))])) print (‘sql %s args %s‘ % (sql, str(args))) return _update(sql,*args)create_engine(user=‘root‘,password=‘z5201314‘,database=‘test‘)u1 = select_one(‘select * from user where id=?‘,1)print ‘u1‘print u1print ‘start selet()...‘u2 = select(‘select * from user‘)for item in u2: print (‘%s %s‘ % (item.name,item.id))print ‘name:%s id: %s‘ % (u1.name,u1.id)
Python学习 - 编写自己的ORM(2)
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