Making a list subclass hashable [closed]

问题

I want to derive a class from list, add a few instance attributes to it, and make it hashable. What is a good (fast and neat) way to do it?

UPDATE:

I deleted a lengthy explanation of a use case. I also moved a related but separate issue into a different question.

回答1:

This code is fine. You're making a copy of the list, which could be a bit slow.

def __hash__(self):
    return hash(tuple(self.list_attribute))

You have several options if you want to be faster.

Store list_attribute as a tuple, not a list (after it is fully constructed)
Compute the hash once at init time and store the hash value. You can do this because your class is immutable, so the hash will never change.
Write your own hash function. Here's the hash function for tuple, do something similar.

回答2:

You can apply tuple to self:

class State(list):
    def __hash__(self):
        return hash((self.some_attribute, tuple(self)))

tuple-ing self takes about half the time of the whole hashing process:

from timeit import timeit

setup = "from __main__ import State; s = State(range(1000)); s.some_attribute = 'foo'"
stmt = "hash(s)"
print(timeit(stmt=stmt, setup=setup, number=100000))

setup = "r = list(range(1000))"
stmt = "tuple(r)"
print(timeit(stmt=stmt, setup=setup, number=100000))

prints

0.9382011891054844
0.3911763069244216

回答3:

This is more of a comment than an answer, but it's too long to be a comment. This is how one can accesses instance attributes from inside __new__:

class Data(tuple):
    def __new__(klass, arg):
        data_inst = tuple.__new__(klass, arg)
        data_inst.min = min(data_inst)
        data_inst.max = max(data_inst)
        return data_inst

>>> d = Data([1,2,3,4])
>>> d
(1, 2, 3, 4)
>>> d.min
1
>>> d.max
4
>>> d1 = Data([1,2,3,4,5,6])
>>> d1.max
6
>>> d.max
4

来源：https://stackoverflow.com/questions/10253783/making-a-list-subclass-hashable

标签

python

list

python-3.x

immutability

hashable