Python type, isinstance, and metaclass applications

Python type, isinstance, and metaclass applications

In Python, you can use the built-in function type to view the object type. isinstance can view that an object is a class instance. Dynamic classes can be implemented through type and dynamic classes can be implemented through metaclass.

Type () and isinstance () determine the object type

import typesclass Hello():    def hello(self, name='World'):        print('hello %s' % name)h = Hello()type(h) == types.InstanceType   //Truetype(Hello)==types.ClassType    //Truetype(isinstance)==types.BuiltinFunctionType //Trueisinstance(h, Hello)        //Truelst = [1, 2, 3]isinstance(lst, (int, str, list))   //True

Type () can be used to obtain the object type. The type is defined in the types module.
Isinstance (object, class-or-type-or-tuple)-> bool determines that an object is an instance of a class or subclass.

Type () implements dynamic classes

Another function of type () is to dynamically create a type.

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

Three parameters of type:
1) class Name;
2) A set of inherited parent classes. Note that Python supports multiple inheritance. If there is only one parent class, do not forget the single-element method of tuple;
3) The method name of the class is bound to the function. Here we bind the function fn to the method name hello.

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 Integer Field (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 mapping S. iterkeys (): attrs. pop (k) attrs ['_ ings _'] = mappings # Save the ing between attributes and columns. attrs ['_ table _'] = name # assume that the table name and class name are consistent. 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] counter t 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 ()