python type、isinstance和metaclass的应用

python type、isinstance和metaclass的应用

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

import types

class Hello():
    def hello(self, name='World'):
        print('hello %s' % name)

h = Hello()
type(h) == types.InstanceType   //True
type(Hello)==types.ClassType    //True
type(isinstance)==types.BuiltinFunctionType //True

isinstance(h, Hello)        //True

lst = [1, 2, 3]
isinstance(lst, (int, str, list))   //True

type()可以得出对象的类型，类型定义在types module中。
isinstance(object, class-or-type-or-tuple) -> bool判断一个对象是某个类或子类的实例。

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。

metaclass

#!/usr/bin/env python
# -*- coding: utf-8 -*-

' Simple ORM using metaclass '

__author__ = 'Michael Liao'

class Field(object):
    def __init__(self, name, column_type):
        self.name = name
        self.column_type = column_type
    def __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] = v
        for 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__ = 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] = value

    def 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()