How To Provide Additional Initialization For A Subclass Of Namedtuple?
Solution 1:
edit for 2017:turns out namedtuple
isn't a great idea. attrs is the modern alternative.
classEdge(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__
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
has to be passed twice to the super
call on __new__
because __new__
is, for historical/odd reasons implicitly a staticmethod
.
Solution 2:
The code in the question could benefit from a super call in the __init__
in case it ever gets subclassed in a multiple inheritance situation, but otherwise is correct.
classEdge(EdgeBase):
def__init__(self, left, right):
super(Edge, self).__init__(left, right)
self._hash = hash(self.left) * hash(self.right)
def__hash__(self):
return self._hash
While tuples are readonly only the tuple parts of their subclasses are readonly, other properties may be written as usual which is what allows the assignment to _hash regardless of whether it's done in __init__
or __new__
. You can make the subclass fully readonly by setting it's __slots__
to (), which has the added benefit of saving memory, but then you wouldn't be able to assign to _hash.
Solution 3:
In Python 3.7+, you can now use dataclasses to build hashable classes with ease.
Code
Assuming int
types of left
and right
, we use the default hashing via unsafe_hash
keyword:
import dataclasses as dc
@dc.dataclass(unsafe_hash=True)classEdge:
left: int
right: inthash(Edge(1, 2))
# 3713081631934410656
Now we can use these (mutable) hashable objects as elements in a set or (keys in a dict).
{Edge(1, 2), Edge(1, 2), Edge(2, 1), Edge(2, 3)}
# {Edge(left=1, right=2), Edge(left=2, right=1), Edge(left=2, right=3)}
Details
We can alternatively override the __hash__
function:
@dc.dataclassclassEdge:
left: int
right: intdef__post_init__(self):
# Add custom hashing function here
self._hash = hash((self.left, self.right)) # emulates defaultdef__hash__(self):
return self._hashhash(Edge(1, 2))
# 3713081631934410656
Expanding on @ShadowRanger's comment, the OP's custom hash function is not reliable. In particular, the attribute values can be interchanged, e.g. hash(Edge(1, 2)) == hash(Edge(2, 1))
, which is likely unintended.
Note, the name "unsafe" suggests the default hash will be used despite being a mutable object. This may be undesired, particularly within a dict expecting immutable keys. Immutable hashing can be turned on with the appropriate keywords. See also more on hashing logic in dataclasses and a related issue.
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