Python - 动手写个ORM

Python - 动手写个ORM

任务：

字段类型类

class Field(object):
"""docstring for Field"""

def __init__(self, field_type, default, max_length,  * arg):
super().__init__()
self.default = default
self.max_length = max_length
self.field_type = field_type

class CharField(Field):
"""docstring for CharField"""

def __init__(self, max_length=100, *arg):
super().__init__('char', '', max_length)

class DateField(Field):
"""docstring for DateField"""

def __init__(self, default='now', *arg):
if(default == 'now'):
from datetime import datetime
self.default = datetime.now().strftime('%Y-%m-%d %H:%M:%S')
else:
self.default = default
super().__init__('datetime', self.default, None)

class IntField(Field):
"""docstring for IntField"""

def __init__(self, default=0, *arg):
super().__init__('int', 0, 8)

很关键的MetaClass

class ModelMetaClass(type):
"""docstring for ModelMetaClass"""

def __new__(cls, name, bases, attrs):
print('2. ModelMetaClass __new__')
mapping = {}
# mapping = dict()
for k, v in attrs.items():
if(isinstance(v, Field)):
mapping[k] = v
for k, v in mapping.items():
del attrs[k]

# 挂到实例上
# attrs['mapping'] = mapping
cls_obj = type.__new__(cls, name, bases, attrs)

# 这里为了测试效果简单的将需要的信息添加到类的动态属性上
cls_obj.mapping = mapping
cls_obj.table = name

return cls_obj

# 如果子类中指定metaclass那么必须从父类的meta继承以维持正确的调用顺序
class NewsMetaClass(ModelMetaClass):
"""docstring for NewsMetaClass"""
print('1. NewsMetaClass __new__')

def __new__(cls, name, bases, attrs):
# do anything you want

# attrs['create'] = NewsMetaClass.create

clsobj = super().__new__(cls, name, bases, attrs)
return clsobj

Model类, 注意

metaclass

的设定

class Model(object, metaclass=ModelMetaClass):
"""docstring for Model"""

def __init__(self, *arg):
super(Model, self).__init__()
self.arg = arg

@classmethod
def create(cls, **kwargs):
fields = []
params = []
args = []

for k, v in cls.mapping.items():
fields.append(k)
params.append('?')
args.append(kwargs[k])

sql = 'insert into %s (%s) values(%s)' % (cls.table, ','.join(fields), ','.join(params))
param = ','.join(args)
print('SQL: %s' % (sql))
print('args: %s' % param)
# exec the sql

class News(Model, metaclass=NewsMetaClass):
"""docstring for News"""
title = CharField(max_length=100)
content = CharField(max_length=500)
publish_date = DateField(default="now")
read_count = IntField(default=0)

测试效果

from datetime import datetime
News.create(title='test', content='asdf', read_count='0', publish_date=datetime.now().strftime('%Y-%m-%d %H:%M:%S'))

# SQL: insert into News (publish_date,content,title,read_count) values(?,?,?,?)
# args: 2016-05-16 22:39:54,asdf,test,0