python3元类简介（metaclass）

在Python中可以用内置函数type查看对象的类型，isinstance查看某个对象是某个类实例，通过type可以实现动态类，以及通过metaclass实现动态类

type()与isinstance()判断对象类型

Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:54:40) [MSC v.1900 64 bit (AMD64)]
Type 'copyright', 'credits' or 'license' for more information
IPython 6.2.1 -- An enhanced Interactive Python. Type '?' for help.In [1]: from selectors import SelectSelectorIn [2]: type(SelectSelector)
Out[2]: abc.ABCMetaIn [3]: from abc import ABCMetaIn [4]: type(ABCMeta)
Out[4]: typeIn [5]: ss = SelectSelector()In [6]: type(ss)
Out[6]: selectors.SelectSelectorIn [7]: from datetime import dateIn [8]: now = date.today()In [9]: type(now)
Out[9]: datetime.dateIn [10]: type(date)
Out[10]: typeIn [16]: isinstance(SelectSelector, ABCMeta)
Out[16]: TrueIn [17]: isinstance(ABCMeta, type)
Out[17]: TrueIn [18]: isinstance(ss, SelectSelector)
Out[18]: True

type(object) -> the object's type 获取对象的类型
isinstance(obj, class_or_tuple 判断对象是否是某一class的实例或者一组class其中一个的实例

通过上面的测试可以得出的结论

类的实例的类型是类，如now是date类型
一个类如果没有metaclass，则该类的类型是type，（默认的），如date类的类型是type
一个类如果有metaclass（其基类有也可以），则该类的类型为其metaclass类，如SelectSelector类的类型为ABCMeta
date是type的实例，type是哪个？
metaclass是什么？为啥那么特殊？作用几个？

instance 				class				metaclassinstance of			instance of

type()实现动态类

type()另一个功能是动态的创建一个类型

>>> def fn(self, name='world'):
...     print('hello %s' % name)
...
>>> HH = type('Hello', (object,),dict(hello=fn))
>>> h = HH()
>>> h.hello()
hello world

type()的三个参数：
1）class的名称；
2）继承的父类集合，注意Python支持多重继承，如果只有一个父类，别忘了tuple的单元素写法；
3）class的方法名称与函数绑定，这里我们把函数fn绑定到方法名hello上。

类是一个对象

type 可以创建类
type 创建的对象拥有创建对象的能力(也就是类)
type 就是 Python 中所有类的元类（metaclass）

metaclass

元类并不是某一个类的名字，它是一个概念，是一种Python的思想。

python2中的用法

#!/usr/bin/env python2
# -*- coding: utf-8 -*-' Simple ORM using metaclass '__author__ = 'Michael Liao'class Field(object):def __init__(self, name, column_type):self.name = nameself.column_type = column_typedef __str__(self):return '<%s:%s>' % (self.__class__.__name__, self.name)class StringField(Field):def __init__(self, name):super(StringField, self).__init__(name, 'varchar(100)')class IntegerField(Field):def __init__(self, name):super(IntegerField, self).__init__(name, 'bigint')class ModelMetaclass(type):def __new__(cls, name, bases, attrs):if name=='Model':return type.__new__(cls, name, bases, attrs)print('Found model: %s' % name)mappings = dict()for k, v in attrs.iteritems():if isinstance(v, Field):print('Found mapping: %s ==> %s' % (k, v))mappings[k] = vfor k in mappings.iterkeys():attrs.pop(k)attrs['__mappings__'] = mappings # 保存属性和列的映射关系attrs['__table__'] = name # 假设表名和类名一致return type.__new__(cls, name, bases, attrs)class Model(dict):__metaclass__ = ModelMetaclassdef __init__(self, **kw):super(Model, self).__init__(**kw)def __getattr__(self, key):try:return self[key]except KeyError:raise AttributeError(r"'Model' object has no attribute '%s'" % key)def __setattr__(self, key, value):self[key] = valuedef save(self):fields = []params = []args = []for k, v in self.__mappings__.iteritems():fields.append(v.name)params.append('?')args.append(getattr(self, k, None))sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(params))print('SQL: %s' % sql)print('ARGS: %s' % str(args))# testing code:class User(Model):id = IntegerField('uid')name = StringField('username')email = StringField('email')password = StringField('password')u = User(id=12345, name='Michael', email='test@orm.org', password='my-pwd')
u.save()

python3中的应用

#!/usr/bin/env python3
# -*- coding: utf-8 -*-' Simple ORM using metaclass '__author__ = 'Michael Liao'class Field(object):def __init__(self, name, column_type):self.name = nameself.column_type = column_typedef __str__(self):return '<%s:%s>' % (self.__class__.__name__, self.name)class StringField(Field):def __init__(self, name):super(StringField, self).__init__(name, 'varchar(100)')class IntegerField(Field):def __init__(self, name):super(IntegerField, self).__init__(name, 'bigint')class ModelMetaclass(type):def __new__(cls, name, bases, attrs):if name=='Model':return type.__new__(cls, name, bases, attrs)print('Found model: %s' % name)mappings = dict()for k, v in attrs.items():if isinstance(v, Field):print('Found mapping: %s ==> %s' % (k, v))mappings[k] = vfor k in mappings.keys():attrs.pop(k)attrs['__mappings__'] = mappings # 保存属性和列的映射关系attrs['__table__'] = name # 假设表名和类名一致return type.__new__(cls, name, bases, attrs)class Model(dict, metaclass = ModelMetaclass):def __init__(self, **kw):super(Model, self).__init__(**kw)def __getattr__(self, key):try:return self[key]except KeyError:raise AttributeError(r"'Model' object has no attribute '%s'" % key)def __setattr__(self, key, value):self[key] = valuedef save(self):fields = []params = []args = []for k, v in self.__mappings__.items():fields.append(v.name)params.append('?')args.append(getattr(self, k, None))sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(params))print('SQL: %s' % sql)print('ARGS: %s' % str(args))# testing code:class User(Model):id = IntegerField('uid')name = StringField('username')email = StringField('email')password = StringField('password')u = User(id=12345, name='Michael', email='test@orm.org', password='my-pwd')
u.save()