Python3 ORM hacking

#!/usr/bin/env python3
# -*- coding: utf- -*-

#
#                        Python3 ORM hacking
# 说明：
#     之前分析了一个Python2 ORM的源代码，这次分析一个Python3的源代码，在写法上
# 还是又挺大的区别的。
#                                        2016-10-22 深圳 南山平山村 曾剑锋
#
# 源码：
#     https://github.com/michaelliao/awesome-python3-webapp/tree/day-03
#
# 参考文章：
#     . python logging模块使用教程
#         http://www.jianshu.com/p/feb86c06c4f4
#     . Python async/await入门
#         https://ipfans.github.io/2015/08/introduction-to-async-and-await/
#     . 浅析python的metaclass
#         http://jianpx.iteye.com/blog/908121
#     . Why I got ignored exception when I use aiomysql in python 3.5 #
#         https://github.com/aio-libs/aiomysql/issues/59
# 

__author__ = 'Michael Liao'

import asyncio, logging

import aiomysql

# SQL日志打印输出模板
def log(sql, args=()):
    logging.info('SQL: %s' % sql)

# 创建数据库连接池
async def create_pool(loop, **kw):
    logging.info('create database connection pool...')
    # 标记__pool为文件内全局变量，在其他函数内可以直接访问
    global __pool
    __pool = await aiomysql.create_pool(
        host=kw.get('host', 'localhost'),
        port=kw.get('port', ),
        user=kw['user'],
        password=kw['password'],
        db=kw['db'],
        charset=kw.get('charset', 'utf8'),
        autocommit=kw.get('autocommit', True),
        maxsize=kw.get('maxsize', ),
        minsize=kw.get('minsize', ),
        loop=loop
    )

# 数据库查询
async def select(sql, args, size=None):
    # 输出SQL日志信息
    log(sql, args)
    global __pool
    # 从连接池中获取连接,aysnc是异步获取连接
    async with __pool.get() as conn:
        async with conn.cursor(aiomysql.DictCursor) as cur:
            # 合成实际的SQL
            await cur.execute(sql.replace('?', '%s'), args or ())
            # 根据size来获取数据多少行记录
            if size:
                rs = await cur.fetchmany(size)
            else:
                rs = await cur.fetchall()
        # 给出获取到的信息条数
        logging.info('rows returned: %s' % len(rs))
        return rs

# 数据库直接执行SQL
async def execute(sql, args, autocommit=True):
    log(sql)
    async with __pool.get() as conn:
        # 如果不是自动提交
        if not autocommit:
            await conn.begin()
        try:
            async with conn.cursor(aiomysql.DictCursor) as cur:
                await cur.execute(sql.replace('?', '%s'), args)
                # 返回的执行SQL后有效行数,从代码上可以看出，这部分主要是执行更新、插入、删除等SQL语句
                affected = cur.rowcount
            # 完成提交工作
            if not autocommit:
                await conn.commit()
        except BaseException as e:
            # 出现问题，回滚
            if not autocommit:
                await conn.rollback()
            raise       # 直接再次抛出异常
        return affected # 返回有效行数

# 合成可替代参数字符串，先使用'?'代替'%s'
def create_args_string(num):
    L = []
    for n in range(num):
        L.append('?')
    return ', '.join(L)

# 对应数据库中每一个字段的一个域的基类
class Field(object):

    # 域名、域类型、是否是主键、默认值
    def __init__(self, name, column_type, primary_key, default):
        self.name = name
        self.column_type = column_type
        self.primary_key = primary_key
        self.default = default

    # 重写默认输出的str函数
    def __str__(self):
        return '<%s, %s:%s>' % (self.__class__.__name__, self.column_type, self.name)

# 字符串类型的域
class StringField(Field):

    def __init__(self, name=None, primary_key=False, default=None, ddl='varchar(100)'):
        super().__init__(name, ddl, primary_key, default)

# Boolean类型的域
class BooleanField(Field):

    def __init__(self, name=None, default=False):
        super().__init__(name, 'boolean', False, default)

# 整形类型的域
class IntegerField(Field):

    def __init__(self, name=None, primary_key=False, default=):
        super().__init__(name, 'bigint', primary_key, default)

# 浮点类型的域
class FloatField(Field):

    def __init__(self, name=None, primary_key=False, default=0.0):
        super().__init__(name, 'real', primary_key, default)

# 文本类型的域
class TextField(Field):

    def __init__(self, name=None, default=None):
        super().__init__(name, 'text', False, default)

# MVC中的Model的元类，主要用于自动生成映射(map)类
class ModelMetaclass(type):

    # name: 类的名字
    # bases: 基类，通常是tuple类型
    # attrs: dict类型，就是类的属性或者函数
    def __new__(cls, name, bases, attrs):
        # 过滤掉Model类直接生成的实例类
        if name=='Model':
            return type.__new__(cls, name, bases, attrs)
        # 从类的属性中获取__table__，其实也就是于数据库对应的表名，如果不存在那么就是等于类名
        tableName = attrs.get('__table__', None) or name
        logging.info('found model: %s (table: %s)' % (name, tableName))
        # 创建映射字典
        mappings = dict()
        # 域list
        fields = []
        # 主键标记
        primaryKey = None
        # 获取类中的所有的键值对
        for k, v in attrs.items():
            # 选择Field类型实例的属性作为映射键值
            if isinstance(v, Field):
                logging.info('  found mapping: %s ==> %s' % (k, v))
                # 将当前的键值对放入mapping中
                mappings[k] = v
                if v.primary_key:
                    # 防止出现两个、两个以上的主键
                    if primaryKey:
                        raise StandardError('Duplicate primary key for field: %s' % k)
                    primaryKey = k
                else:
                    # 将key添加进入fields中，也就是映射类中的属性和数据库中的表的域, 这里面不包含主键
                    fields.append(k)

        # 前面可能没有找到主键，提示一下
        if not primaryKey:
            raise StandardError('Primary key not found.')
        # 删除这些类属性 防止访问实例属性的时候发生错误，因为实例属性优先级大于类属性
        for k in mappings.keys():
            attrs.pop(k)

        escaped_fields = list(map(lambda f: '`%s`' % f, fields))

        attrs['__mappings__'] = mappings        # 保存属性和列的映射关系
        attrs['__table__'] = tableName          # 表名
        attrs['__primary_key__'] = primaryKey   # 主键属性名
        attrs['__fields__'] = fields            # 除主键外的属性名

        # 生成查询SQL
        attrs['__select__'] = 'select `%s`, %s from `%s`' % (primaryKey, ', '.join(escaped_fields), tableName)
        # 生成插入SQL
        attrs['__insert__'] = 'insert into `%s` (%s, `%s`) values (%s)' % (tableName, ', '.join(escaped_fields), primaryKey, create_args_string(len(escaped_fields) + ))
        # 生成更新SQL
        attrs['__update__'] = 'update `%s` set %s where `%s`=?' % (tableName, ', '.join(map(lambda f: '`%s`=?' % (mappings.get(f).name or f), fields)), primaryKey)
        # 生成删除SQL
        attrs['__delete__'] = 'delete from `%s` where `%s`=?' % (tableName, primaryKey)

        # 调用type生成类
        return type.__new__(cls, name, bases, attrs)

# 继承自ModelMetaclass元类、dict的类
class Model(dict, metaclass=ModelMetaclass):

    def __init__(self, **kw):
        super(Model, self).__init__(**kw)

    # 重写get方法
    def __getattr__(self, key):
        try:
            return self[key]
        except KeyError:
            raise AttributeError(r"'Model' object has no attribute '%s'" % key)

    # 重写set方法
    def __setattr__(self, key, value):
        self[key] = value

    # 重写get方法
    def getValue(self, key):
        return getattr(self, key, None)

    # 获取值，当不存在的时候获取的是默认值
    def getValueOrDefault(self, key):
        value = getattr(self, key, None)
        if value is None:
            field = self.__mappings__[key]
            if field.default is not None:
                value = field.default() if callable(field.default) else field.default
                logging.debug('using default value for %s: %s' % (key, str(value)))
                setattr(self, key, value)
        return value

    @classmethod
    async def findAll(cls, where=None, args=None, **kw):
        ' find objects by where clause. '
        # 获取元类自动生成的SQL语句,并根据当前的参数，继续合成
        sql = [cls.__select__]
        if where:
            sql.append('where')
            sql.append(where)
        if args is None:
            args = []
        orderBy = kw.get('orderBy', None)
        if orderBy:
            sql.append('order by')
            sql.append(orderBy)
        limit = kw.get('limit', None)
        if limit is not None:
            sql.append('limit')
            if isinstance(limit, int):
                sql.append('?')
                args.append(limit)
            elif isinstance(limit, tuple) and len(limit) == :
                sql.append('?, ?')
                args.extend(limit)
            else:
                raise ValueError('Invalid limit value: %s' % str(limit))
        # 直接调用select函数来处理, 这里是等待函数执行完成函数才能返回
        rs = await select(' '.join(sql), args)
        # 该类本身是字典，自己用自己生成新的实例，里面的阈值正好也是需要查询
        return [cls(**r) for r in rs]

    @classmethod
    async def findNumber(cls, selectField, where=None, args=None):
        ' find number by select and where. '
        # 这里的 _num_ 什么意思？别名？ 我估计是mysql里面一个记录实时查询结果条数的变量
        sql = ['select %s _num_ from `%s`' % (selectField, cls.__table__)]
        if where:
            sql.append('where')
            sql.append(where)
        rs = await select(' '.join(sql), args, )
        if len(rs) == :
            return None
        return rs[]['_num_']

    @classmethod
    async def find(cls, pk):
        ' find object by primary key. '
        # 通过主键查找对象, 如果不存在，那么就返回None
        rs = await select('%s where `%s`=?' % (cls.__select__, cls.__primary_key__), [pk], )
        if len(rs) == :
            return None
        return cls(**rs[])

    # 插入语句对应的方法
    async def save(self):
        args = list(map(self.getValueOrDefault, self.__fields__))
        args.append(self.getValueOrDefault(self.__primary_key__))
        rows = await execute(self.__insert__, args)
        if rows != :
            logging.warn('failed to insert record: affected rows: %s' % rows)

    # 更新语句对应的方法
    async def update(self):
        args = list(map(self.getValue, self.__fields__))
        args.append(self.getValue(self.__primary_key__))
        rows = await execute(self.__update__, args)
        if rows != :
            logging.warn('failed to update by primary key: affected rows: %s' % rows)

    # 删除语句对应的方法
    async def remove(self):
        args = [self.getValue(self.__primary_key__)]
        rows = await execute(self.__delete__, args)
        if rows != :
            logging.warn('failed to remove by primary key: affected rows: %s' % rows)