对 Python set 类进行子类化(添加新的实例变量)的正确(或最佳)方法是什么?
我正在实现一个与集合几乎相同的对象,但需要一个额外的实例变量,因此我对内置集合对象进行子类化。 确保复制我的对象之一时复制此变量的值的最佳方法是什么?
使用旧的 set 模块,以下代码可以完美运行:
import sets
class Fooset(sets.Set):
def __init__(self, s = []):
sets.Set.__init__(self, s)
if isinstance(s, Fooset):
self.foo = s.foo
else:
self.foo = 'default'
f = Fooset([1,2,4])
f.foo = 'bar'
assert( (f | f).foo == 'bar')
但是使用内置的 set 模块则不起作用。
我能看到的唯一解决方案是重写返回复制的集合对象的每个方法...在这种情况下,我可能也不必费心对集合对象进行子类化。 当然有一个标准方法可以做到这一点吗?
(澄清一下,以下代码不起作用(断言失败)
class Fooset(set):
def __init__(self, s = []):
set.__init__(self, s)
if isinstance(s, Fooset):
self.foo = s.foo
else:
self.foo = 'default'
f = Fooset([1,2,4])
f.foo = 'bar'
assert( (f | f).foo == 'bar')
:)
I'm implementing an object that is almost identical to a set, but requires an extra instance variable, so I am subclassing the built-in set object. What is the best way to make sure that the value of this variable is copied when one of my objects is copied?
Using the old sets module, the following code worked perfectly:
import sets
class Fooset(sets.Set):
def __init__(self, s = []):
sets.Set.__init__(self, s)
if isinstance(s, Fooset):
self.foo = s.foo
else:
self.foo = 'default'
f = Fooset([1,2,4])
f.foo = 'bar'
assert( (f | f).foo == 'bar')
but this does not work using the built-in set module.
The only solution that I can see is to override every single method that returns a copied set object... in which case I might as well not bother subclassing the set object. Surely there is a standard way to do this?
(To clarify, the following code does not work (the assertion fails):
class Fooset(set):
def __init__(self, s = []):
set.__init__(self, s)
if isinstance(s, Fooset):
self.foo = s.foo
else:
self.foo = 'default'
f = Fooset([1,2,4])
f.foo = 'bar'
assert( (f | f).foo == 'bar')
)
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我最喜欢的包装内置集合方法的方法:
本质上与您在自己的答案中所做的事情相同,但位置较少。 如果您也想对列表和字典执行相同的操作,那么放入元类也很容易。
My favorite way to wrap methods of a built-in collection:
Essentially the same thing you're doing in your own answer, but with fewer loc. It's also easy to put in a metaclass if you want to do the same thing with lists and dicts as well.
我认为推荐的方法不是直接从内置
set
子类化,而是使用 抽象基类Set
位于 collections.abc。使用 ABC Set 为您提供了一些免费的混合方法,因此您可以通过仅定义
__contains__()
、__len__()
和来获得最小的 Set 类>__iter__()
。 如果您想要一些更好的设置方法,例如intersection()
和difference()
,您可能必须包装它们。这是我的尝试(这个恰好是类似 freezeset 的,但您可以从
MutableSet
继承来获取可变版本):I think that the recommended way to do this is not to subclass directly from the built-in
set
, but rather to make use of the Abstract Base ClassSet
available in collections.abc.Using the ABC Set gives you some methods for free as a mix-in so you can have a minimal Set class by defining only
__contains__()
,__len__()
and__iter__()
. If you want some of the nicer set methods likeintersection()
anddifference()
, you probably do have to wrap them.Here's my attempt (this one happens to be a frozenset-like, but you can inherit from
MutableSet
to get a mutable version):遗憾的是,set 不遵循规则,并且不会调用
__new__
来创建新的set
对象,即使它们保留了类型。 这显然是 Python 中的一个错误(问题 #1721812,在 2.x 中不会修复)顺序)。 如果不调用创建 X 对象的type
对象,则永远无法获取类型 X 的对象! 如果set.__or__
不打算调用__new__
,它就必须返回set
对象而不是子类对象。但实际上,注意到上面 nosklo 的帖子,你原来的行为没有任何意义。
Set.__or__
运算符不应该重用任何一个源对象来构造其结果,它应该创建一个新的对象,在这种情况下,它的foo
应该是 <代码>“默认”!因此,实际上,任何执行此操作的人都应该必须重载这些运算符,以便他们知道使用了
foo
的哪个副本。 如果它不依赖于组合的 Foosets,您可以将其设置为默认类,在这种情况下它将受到尊重,因为新对象认为它是子类类型。我的意思是,如果你这样做的话,你的例子就会起作用:
Sadly, set does not follow the rules and
__new__
is not called to make newset
objects, even though they keep the type. This is clearly a bug in Python (issue #1721812, which will not be fixed in the 2.x sequence). You should never be able to get an object of type X without calling thetype
object that creates X objects! Ifset.__or__
is not going to call__new__
it is formally obligated to returnset
objects instead of subclass objects.But actually, noting the post by nosklo above, your original behavior does not make any sense. The
Set.__or__
operator should not be reusing either of the source objects to construct its result, it should be whipping up a new one, in which case itsfoo
should be"default"
!So, practically, anyone doing this should have to overload those operators so that they would know which copy of
foo
gets used. If it is not dependent on the Foosets being combined, you can make it a class default, in which case it will get honored, because the new object thinks it is of the subclass type.What I mean is, your example would work, sort of, if you did this:
<代码>设置1 | set2 是一个不会修改现有
set
的操作,而是返回一个新的set
。 新的集
被创建并返回。 没有办法让它自动将任意属性从一个或两个set
复制到新创建的set
,而无需自定义|
通过定义__or__
方法自己操作 。印刷:
set1 | set2
is an operation that won't modify either existingset
, but return a newset
instead. The newset
is created and returned. There is no way to make it automatically copy arbritary attributes from one or both of theset
s to the newly createdset
, without customizing the|
operator yourself by defining the__or__
method.Prints:
看起来 set 绕过了 c 代码。 然而,您将结束
Fooset
的实例,它只是没有机会复制该字段。除了覆盖返回新集合的方法之外,我不确定在这种情况下您可以做太多事情。 Set 显然是为了一定的速度而构建的,c 中的很多工作也是如此。
It looks like set bypasses
__init__
in the c code. However you will end an instance ofFooset
, it just won't have had a chance to copy the field.Apart from overriding the methods that return new sets I'm not sure you can do too much in this case. Set is clearly built for a certain amount of speed, so does a lot of work in c.
我试图回答阅读它的问题:“如何使“集合”运算符的返回值成为我的集合子类的类型。忽略给定类的详细信息以及示例是否我从我自己的问题来到这里,如果我的阅读正确的话,
这个答案与其他一些答案不同,如下所示:
库代码,可以放在项目或模块中的任何位置:
要将其与集合一起使用,我们需要返回一个新实例的方法列表:
并且我们可以使用它与我们的类:
我不会详细说明该类的特殊之处。
我已经用以下几行测试了代码:
其中 print:
有一种情况,结果不是最佳的。 在这种情况下,运算符与类的实例一起使用作为右侧操作数,并且将内置“集合”的实例作为第一个操作数。 我不喜欢这个,但我相信这个问题对于我见过的所有提议的解决方案都是常见的。
我还考虑过提供一个使用 collections.abc.Set 的示例。
虽然可以这样做:
我不确定它是否具有 @bjmc 所想到的好处,或者它“免费”为您提供的“一些方法”是什么。
该解决方案的目标是使用基类来完成工作并返回子类的实例。 使用成员对象来完成工作的解决方案可能会以类似的方式生成。
I am trying to answer the questions reading it as: "How can I make the return values of the operators of "set" to be of the type of my subclass of set. Ignoring the details of the given class and whether or not the example is broken to begin with. I came here from my own question which would be a duplicate, if my reading is correct.
This answer differs from some of the other answers as follows:
The library code, which can be put anywhere in the project or a module:
To use it with a set, we need the list of methods, that return a new instance:
and we can use it with our class:
I am sparing the details of what could be special about this class.
I have tested the code with the following lines:
which print:
There is one case, in which the result is not optimal. This is, where the operator is used with an instance of the class as right hand operand and an instance of the builtin 'set' as first. I don't like this, but I believe this problem is common to all proposed solutions I have seen.
I have also thought of providing an example, where the collections.abc.Set is used.
While it could be done like this:
I am not sure, whether it comes with the benefits, that @bjmc had in mind, or what the "some methods" are, that it gives you "for free".
This solution is targeted at using a base class to do the work and return instances of the subclass. A solution, that uses a member object to do the work could probably be generated in a similar way.
使用装饰器概括 Matthew Marshall 响应而不添加运行时检查,这会影响性能:
干净的代码:
Generalizing Matthew Marshall response using a decorator without adding runtime checking, which would impact performance:
clean code:
假设其他答案是正确的,并且重写所有方法是执行此操作的唯一方法,这是我尝试采用一种适度优雅的方法来执行此操作。 如果添加更多的实例变量,只需要改变一段代码。 不幸的是,如果将新的二元运算符添加到集合对象中,则此代码将中断,但我认为没有办法避免这种情况。 欢迎评论!
Assuming the other answers are correct, and overriding all the methods is the only way to do this, here's my attempt at a moderately elegant way of doing this. If more instance variables are added, only one piece of code needs to change. Unfortunately if a new binary operator is added to the set object, this code will break, but I don't think there's a way to avoid that. Comments welcome!
对我来说,在 Win32 上使用 Python 2.5.2 可以完美地工作。 使用您的类定义和以下测试:
我得到这个输出,这是我所期望的:
For me this works perfectly using Python 2.5.2 on Win32. Using you class definition and the following test:
I get this output, which is what I expect: