如何为 namedtuple 的子类提供额外的初始化?
问题描述
假设我有一个像这样的 namedtuple
:
Suppose I have a namedtuple
like this:
EdgeBase = namedtuple("EdgeBase", "left, right")
我想为此实现一个自定义哈希函数,所以我创建了以下子类:
I want to implement a custom hash-function for this, so I create the following subclass:
class Edge(EdgeBase):
def __hash__(self):
return hash(self.left) * hash(self.right)
由于对象是不可变的,我希望哈希值只计算一次,所以我这样做:
Since the object is immutable, I want the hash-value to be calculated only once, so I do this:
class Edge(EdgeBase):
def __init__(self, left, right):
self._hash = hash(self.left) * hash(self.right)
def __hash__(self):
return self._hash
这似乎有效,但我真的不确定 Python 中的子类化和初始化,尤其是元组.这个解决方案有什么陷阱吗?有推荐的方法吗?好吗?提前致谢.
This appears to be working, but I am really not sure about subclassing and initialization in Python, especially with tuples. Are there any pitfalls to this solution? Is there a recommended way how to do this? Is it fine? Thanks in advance.
解决方案
2017 年 结果表明 namedtuple
不是一个好主意.attrs 是现代替代方案.
edit for 2017: turns out namedtuple
isn't a great idea. attrs is the modern alternative.
class Edge(EdgeBase):
def __new__(cls, left, right):
self = super(Edge, cls).__new__(cls, left, right)
self._hash = hash(self.left) * hash(self.right)
return self
def __hash__(self):
return self._hash
__new__
是你想在这里调用的,因为元组是不可变的.不可变对象在 __new__
中创建,然后返回给用户,而不是在 __init__
中填充数据.
__new__
is what you want to call here because tuples are immutable. Immutable objects are created in __new__
and then returned to the user, instead of being populated with data in __init__
.
cls
必须两次传递给 __new__
上的 super
调用,因为 __new__
对于历史/奇怪的原因隐含了一个 staticmethod
.
cls
has to be passed twice to the super
call on __new__
because __new__
is, for historical/odd reasons implicitly a staticmethod
.
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