Question

这符合我的预期;

class Foo(object):
    name = ''

class Bar1(Foo):
    pass

class Bar2(Foo):
    pass

Bar1.name == Bar2.name # returns True
Bar1.name = 'Bar1'
Bar1.name == Bar2.name # returns False, which is what I want.

这不起作用，但我想要它;

class Foo(object):
    fields = {'name':''}

class Bar1(Foo):
    pass

class Bar2(Foo):
    pass

Bar1.fields['name'] == Bar2.fields['name'] # returns True
Bar1.fields['name'] = 'Bar1'
Bar1.fields['name'] == Bar2.fields['name'] # returns True, which isn't what I want.

似乎子类仍指向主类中指定的相同dict对象，但我希望它们具有唯一的dicts。如果不在每个子类中编写fields = {'name':''}，我该怎么做？

PS-我当然希望使用类级别属性，而不是实例属性，因为我创建的所有实例都将使用此共享＆＃39;字典。

Answer 1

最简单的方法是使用元类（我假设使用Python 2.x语法）：

Identity

使用中：

class FieldsMeta(type):

    def __new__(mcs, name, bases, dict):
        """This controls the creation of each new class."""
        dict.update(fields={'name': ''})
        return type.__new__(mcs, name, bases, dict)


class Parent(object):
    __metaclass__ = FieldsMeta


class Child(Parent):
    pass

参见例如data-model documentation for __new__：

[>>> Parent.fields {'name': ''} >>> Child.fields {'name': ''} >>> Child.fields is Parent.fields False >>> Child.fields['name'] = 'Child' >>> Child.fields {'name': 'Child'} >>> Parent.fields {'name': ''}]也通常在自定义元类中按顺序重写自定义类创建。

以及customizing class creation上的部分。

Answer 2

您可以创建字段setter，它将在分配新值之前复制dict

class Foo(object):
    fields = {'name':''}

    def set_field(self, field, value):
        self.fields = dict(self.fields)
        self.fields[field] = value

class Bar1(Foo):
    pass

class Bar2(Foo):
    pass

Bar1.fields['name'] == Bar2.fields['name'] # returns True
# Bar1.fields['name'] = 'Bar1'
Bar1.set_field(Bar1, 'name', 'Bar1')
print(Bar1.fields['name'] == Bar2.fields['name']) # returns False

如何继承作为dict的class属性并使其唯一？

2 个答案: