网站备案幕布多少钱,wordpress返回上一页插件,服装网站模板,个人网站意义在Python中可以用内置函数type查看对象的类型#xff0c;isinstance查看某个对象是某个类实例#xff0c;通过type可以实现动态类#xff0c;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…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]: Truetype(object) - the objects 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 oftype()实现动态类
type()另一个功能是动态的创建一个类型 def fn(self, nameworld):
... print(hello %s % name)
...HH type(Hello, (object,),dict(hellofn))h HH()h.hello()
hello worldtype()的三个参数 1class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 3class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 Liaoclass 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 nameModel: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(rModel 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(id12345, nameMichael, emailtestorm.org, passwordmy-pwd)
u.save()python3中的应用
#!/usr/bin/env python3
# -*- coding: utf-8 -*- Simple ORM using metaclass __author__ Michael Liaoclass 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 nameModel: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(rModel 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(id12345, nameMichael, emailtestorm.org, passwordmy-pwd)
u.save()参考
Python3 元类metaclass Understanding Python metaclasses
