基础概念:
type类是所有类的元类,所有类可以通过type(name, bases, attrs)
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] = v
for 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