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, logging

class 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] = value
class DBError(Exception):
    pass
class MultiColumnsError(Exception):
    pass
engine = None
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
        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 _wrapper

def _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_connection
def select_one(sql,*args):
    return _select(sql,True,*args)
@with_connection
def select_int(sql,*args):
    d = _select(sql,True,*args)
    if len(d) != 1:
        raise MultoColumnsError('Except only one column.')
    return d.values()[0]
@with_connection
def select(sql,*args):
    global engine
    print type(engine)
    return _select(sql,False,*args)
@with_connection
def _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 u1
print '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)