python元类使用经验总结

    name是新建类的名字，bases是基类的列表，attrs是类方法类变量这些属性

py3创建元类的方法：class NewClass(bases, metaclass=SayMetaClass)

class SayMetaClass(type):def __new__(cls, name, bases, attrs):attrs['say_'+name] = lambda self,value,saying=name: print(saying+','+value+'!')return type.__new__(cls, name, bases, attrs)

元类可以改变类的结构和属性，创建的对象的时候所有的就都生效了；而__new__只能在创建对象的时候根据现有属性进行操作；

上面改变结构后，生成新类的时候，type.__new__可以改成type,super(SayMetaClass,self),super()---py3

orm使用元类实例：

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)

把类里面的field属性提取出来，放在字典里面保存成__mappings__属性，删除原来field类属性

class Model(dict, metaclass=ModelMetaclass):def __init__(self, **kwarg):super(Model, self).__init__(**kwarg)def __getattr__(self, key):try:return self.__mappings__[key]except KeyError:raise AttributeError("'Model' object has no attribute '%s'" % key)def __setattr__(self, key, value):self.__mappings__[key] = value# 模拟建表操作def save(self):fields = []args = []for k, v in self.__mappings__.items():fields.append(v.name)args.append(getattr(self, k, None))sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join([str(i) for i in args]))print('SQL: %s' % sql)print('ARGS: %s' % str(args))

User->Model->ModelBase,继承关系，Model里面放了对数据库操作的方法，ModelBase完成对象向数据库的影射操作

元类可以继承，查找元类的顺序 自己->mro父类->模块->type