Python 3.x 元类不初始化新类
全部!我的代码有一个大问题。我试图编写简单的代码作为理解元编程的简单框架。我还有下一个问题Python 3.x 元类不初始化新类,python-3.x,metaclass,Python 3.x,Metaclass,全部!我的代码有一个大问题。我试图编写简单的代码作为理解元编程的简单框架。我还有下一个问题 doc1 = Document() doc1.id = 20 doc2 = Document() print (doc2.id) # show 20, but it's wrong!!! It should be 0 这是我的代码列表 class BaseModel(type): def __new__(cls, name, bases, attrs, **kwargs): instance
doc1 = Document()
doc1.id = 20
doc2 = Document()
print (doc2.id) # show 20, but it's wrong!!! It should be 0
这是我的代码列表
class BaseModel(type):
def __new__(cls, name, bases, attrs, **kwargs):
instance = super().__new__
parents = [b for b in bases if isinstance(b, BaseModel)]
if not parents:
return instance(cls, name, bases, attrs)
module_ = attrs.pop('__module__')
attrs_ = {'__module__': module_}
classcell_ = attrs.pop('__classcell__', None)
if classcell_ is not None:
attrs_['__classcell__'] = classcell_
for key, value in attrs.items():
if not isinstance(value, Field):
continue
print (value.__dict__)
attrs_[key] = value
instance = instance(cls, name, bases, attrs_, **kwargs)
return instance
class Model(metaclass=BaseModel):
pass
class Field:
def __init__(self, value=0):
self._value = value
def __get__(self, instance, owner):
return self._value
def __set__(self, instance, value):
self._value = value
class Document(Model):
id = Field()
发生什么事了?我认为主要问题是元类中的新方法。我说得对吗?如何修复它?尽管元类不是问题的根源,但您试图用元类完成的工作令人困惑。实现唯一ID不需要元类,只需要精心编制的描述符 在您的例子中,
字段
描述符跟踪单个值,而不是现有的实例和ID对
要解决此问题,您需要保留已获取值的弱引用
import weakref, itertools
class UniqueId:
def __init__(self):
self._values = weakref.WeakKeyDictionary()
def __get__(self, instance, owner):
if instance is None:
return self
else:
return self._values[instance]
def __set__(self, instance, value):
if value not in self._values.values():
self._values[instance] = value
else:
raise ValueError('Unique id {} already taken'.format(value))
def get_unique_id(self):
existing_ids = set(self._values.values())
return next(i for i in itertools.count() if i not in existing_ids)
class Model:
def __new__(cls, *args, **kwargs):
instance = super().__new__(cls, *args, **kwargs)
for name, attr in cls.__dict__.items():
if isinstance(attr, UniqueId):
setattr(instance, name, attr.get_unique_id())
return instance
class Document(Model):
id = UniqueId()
例子
虽然元类不是问题的根源,但您试图用元类完成的工作令人困惑。实现唯一ID不需要元类,只需要精心编制的描述符 在您的例子中,
字段
描述符跟踪单个值,而不是现有的实例和ID对
要解决此问题,您需要保留已获取值的弱引用
import weakref, itertools
class UniqueId:
def __init__(self):
self._values = weakref.WeakKeyDictionary()
def __get__(self, instance, owner):
if instance is None:
return self
else:
return self._values[instance]
def __set__(self, instance, value):
if value not in self._values.values():
self._values[instance] = value
else:
raise ValueError('Unique id {} already taken'.format(value))
def get_unique_id(self):
existing_ids = set(self._values.values())
return next(i for i in itertools.count() if i not in existing_ids)
class Model:
def __new__(cls, *args, **kwargs):
instance = super().__new__(cls, *args, **kwargs)
for name, attr in cls.__dict__.items():
if isinstance(attr, UniqueId):
setattr(instance, name, attr.get_unique_id())
return instance
class Document(Model):
id = UniqueId()
例子
太棒了。谢谢大家!@基里尔:没问题。在StackOverflow上,单击左侧的绿色复选标记,毫不犹豫地接受允许ot解决问题的答案。这有助于其他用户找到有用的答案,并为提问者和回答者提供一些良好的声誉。这太棒了。谢谢大家!@基里尔:没问题。在StackOverflow上,单击左侧的绿色复选标记,毫不犹豫地接受允许ot解决问题的答案。这有助于其他用户找到有用的答案,并为提问者和回答者提供一些良好的声誉。