无法将列表转换为集合，引发“unhashable type: 'list'"错误

问题描述

所以我正在尝试查找列表的所有子列表，这就是我现在所拥有的.我是 Python 新手，我不明白为什么Q3_ans=set(ans)"会引发错误.我曾尝试将列表转换为之前的设置，它可以工作.

So I'm trying to find all sub-lists of a list and here is what I have now. I'm new to Python and I don't understand why " Q3_ans=set(ans)" raises an error. I've tried to convert a list to set before and it works.

def f2(seq):
    '''
    This is the base case of the recursion from function all_sublists
    '''
    assert len(seq)==2
    assert isinstance(x,list)
    a,b=seq
    return [[a],[b],[a,b]]


def all_sublists(x):
    '''
    This function will generate all of the sublists of a list, not including the empty one, using recursion
    '''
    assert isinstance(x,list)
    ans=[]
    for i in range(0,len(x)-1):
        for j in range(1,len(x)):
            temp=[x[i],x[j]]
            temp=[f2(temp)]
            ans.extend(temp)
    Q3_ans=set(ans) 
    return Q3_ans

这是我运行代码时的错误y=[1,2,3,4,5]

Here is the error when I run my code y=[1,2,3,4,5]

all_sublists(y)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-108-f8b1bb0a7001> in <module>
----> 1 all_sublists(y)

<ipython-input-106-84f4f752e98e> in all_sublists(x)
     10             temp=[f2(temp)]
     11             ans.extend(temp)
---> 12     Q3_ans=set(ans)
     13     return Q3_ans

TypeError: unhashable type: 'list'

推荐答案

这是问题的本质:

>>> set([[["a"],["b"],["a","b"]]])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

那么，这是什么意思?

set([iterable])

返回一个新的 set ... 对象，其元素取自可迭代对象.集合的元素必须是可散列的.

Return a new set ... object whose elements are taken from iterable. The elements of a set must be hashable.

可散列

一个对象是可哈希的，如果它有一个永远不会改变的哈希值在其生命周期内(它需要一个 __hash__() 方法)，并且可以与其他对象相比(它需要一个 __eq__() 方法).可散列比较相等的对象必须具有相同的哈希值.

An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() method). Hashable objects which compare equal must have the same hash value.

Hashability 使对象可用作字典键和集合成员，因为这些数据结构在内部使用哈希值.

Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally.

所有 Python 的不可变内置对象都是可散列的；可变的容器(例如列表或字典)不是.

All of Python’s immutable built-in objects are hashable; mutable containers (such as lists or dictionaries) are not.

这里的关键字是可变

The keywords here are mutable

可变对象可以改变它们的值，但保留它们的 id().

Mutable objects can change their value but keep their id().

和 不可变

具有固定值的对象.不可变对象包括数字，字符串和元组.这样的对象是不能改变的.一个新对象有如果必须存储不同的值，则创建.他们玩在需要恒定哈希值的地方发挥重要作用，例如例如作为字典中的键.

An object with a fixed value. Immutable objects include numbers, strings and tuples. Such an object cannot be altered. A new object has to be created if a different value has to be stored. They play an important role in places where a constant hash value is needed, for example as a key in a dictionary.

因此，您不能将 list 用作 set 元素.tuple 可以工作:

So, you can't use a list as set element. A tuple would work:

>>> set([(("a"),("b"),("a","b"))])
{('a', 'b', ('a', 'b'))}

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