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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)