python - 深拷贝有问题吗？

Question

资源

from copy import deepcopy

class Field(object):
    def __init__(self):
        self.errors = []

class BaseForm(object):
    pass

class MetaForm(type):
    def __new__(cls, name, bases, attrs):
        attrs['fields'] = dict([(name, deepcopy(attrs.pop(name))) for name, obj in attrs.items() if isinstance(obj, Field)])
        return type.__new__(cls, name, bases, attrs)

class Form(BaseForm):
    __metaclass__ = MetaForm

class MyForm(Form):
    field1 = Field()

f1 = MyForm()
f1.fields['field1'].errors += ['error msg']

f2 = MyForm()
print f2.fields['field1'].errors

输出

['error msg']

问题

为什么会这样输出？我以为我在修改之前克隆了错误列表，并且它们不应该都引用同一个列表？

Answer 1

通过设置dict fields中的metaclass，您正在创建一个类属性。

您定义的__new__方法只运行一次——在创建类时。

更新

你应该像你一样进行操作，但将其命名attrs为. 然后创建一个执行到被调用的方法。__new___fields__init__deepcopyattributefields

Answer 2

更明确的解决方案：

from copy import deepcopy

class Field(object):
    def __init__(self):
        self.errors = []

class BaseForm(object):
    def __init__(self):
        self.fields = deepcopy(self.fields)

class MetaForm(type):
    def __new__(cls, name, bases, attrs):
        attrs['fields'] = dict([(name, attrs.pop(name)) for name, obj in attrs.items() if isinstance(obj, Field)])
        return type.__new__(cls, name, bases, attrs)

class Form(BaseForm):
    __metaclass__ = MetaForm

class MyForm(Form):
    field1 = Field()

f1 = MyForm()
f1.fields['field1'].errors += ['error msg']

f2 = MyForm()
print f2.fields['field1'].errors

只是将 in 移到了deepcopy，BaseForm.__init__实际上每次实例化a 时都会调用它。MyForm