“追加”而不是覆盖超类实例变量

发布于 2024-12-08 20:27:30 字数 164 浏览 0 评论 0原文

在 Python 中，有没有办法做这样的事情？

class A():
    var = "1, 2, 3"

class B():
    var = ... ", 4"


instance = B()
instance.var # = 1, 2, 3, 4

原文

Is there a way, in Python, to do something like this?

class A():
    var = "1, 2, 3"

class B():
    var = ... ", 4"


instance = B()
instance.var # = 1, 2, 3, 4

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缘字诀 2024-12-15 20:27:30

问题中给出的示例既不使用超类也不使用实例变量。从问题的标题来看，想必您想同时使用两者。下面是它如何工作的示例：

class A(object):
  def __init__(self):
    self.var = "1,2,3"

class B(A):
  def __init__(self):
    super(B,self).__init__()
    self.var += ",4,5"

print A().var
print B().var

请注意，您的示例只是使用类似于命名空间的类将属性分配给不相关的类。这是完全合理的，但不完全符合你的要求：

class A:
  var = "1,2,3"

class B:
  var = A.var + ",4,5"

print A.var
print B.var

The example given in the question uses neither a superclass nor an instance variable. From the title of the question, presumably you wanted to use both. Here's an example of how it would work:

class A(object):
  def __init__(self):
    self.var = "1,2,3"

class B(A):
  def __init__(self):
    super(B,self).__init__()
    self.var += ",4,5"

print A().var
print B().var

Note that your example simply assigns attributes to unrelated classes, using the classes sort-of like namespaces. Which is perfectly reasonable, but not quite what you asked:

class A:
  var = "1,2,3"

class B:
  var = A.var + ",4,5"

print A.var
print B.var

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旧梦荧光笔 2024-12-15 20:27:30

你不能追加，你可以替换，但你可以用包含原始内容的表达式替换：

class A(object):
    var = "1, 2, 3"

class B(A):
    var = A.var + ", 4"


instance = B()
instance.var # = 1, 2, 3, 4

这只是为了简单的用途，但更有趣的解决方案可能是使用描述符；最简单的方法是使用 property，

class A(object):
    var = "1, 2, 3"

class B(A):
    _subvar = ", 4"
    @property
    def var(self):
        return super(B, self).var + self._subvar


instance = B()
instance.var # = 1, 2, 3, 4

它适用于 B 的实例，但不适用于 B 本身（如第一个示例）。要恢复它，您必须实现自己的描述符：

class Appender(object):
    def __init__(self, cls, attr):
        self.cls = cls
        self.attr = attr
    def __get__(self, instance, owner):
        var = super(self.cls, owner).var
        return var + getattr(self.cls, self.attr)

class B(A):
    _realvar = ', 4'
B.var = Appender(B, '_realvar')

它在类上和在类的实例上同样有效

you can't append, you replace, but you can replace by an expression that includes the original:

class A(object):
    var = "1, 2, 3"

class B(A):
    var = A.var + ", 4"


instance = B()
instance.var # = 1, 2, 3, 4

That does it for simple uses, but a more interesting solution might be to use a descriptor; The easy way is to use property

class A(object):
    var = "1, 2, 3"

class B(A):
    _subvar = ", 4"
    @property
    def var(self):
        return super(B, self).var + self._subvar


instance = B()
instance.var # = 1, 2, 3, 4

Which works for instances of B, but not for B itself (as in the first example). To get that back, you must implement your own descriptor:

class Appender(object):
    def __init__(self, cls, attr):
        self.cls = cls
        self.attr = attr
    def __get__(self, instance, owner):
        var = super(self.cls, owner).var
        return var + getattr(self.cls, self.attr)

class B(A):
    _realvar = ', 4'
B.var = Appender(B, '_realvar')

Which works equally well on the class as it does on instances of the class

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