python - 为什么我不能让Python设置不同？

Question

所以我做了一些经验:

>>>{2,3} - {2}
{3}

这个非常好用。

但是，这个好像不行:

>>> {{2,3},{4,3}} - {{4,3}}
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'set'

我认为它应该有效，

因为我使用一组包含两个元素 {2,3} 和 {4,3} 来减去一个元素 {4,3}。

尽管如此，它还是行不通。为什么？

Answer 1

set 不可散列；不能是集合的成员。使用 frozenset相反:

>>> {frozenset({2,3}), frozenset({4,3})} - {frozenset({4,3})}
set([frozenset([2, 3])])

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根据 set / frozenset documentation :

The elements of a set must be hashable. To represent sets of sets, the inner sets must be frozenset objects. If iterable is not specified, a new empty set is returned.

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hashable

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

All of Python’s immutable built-in objects are hashable, while no mutable containers (such as lists or dictionaries) are. Objects which are instances of user-defined classes are hashable by default; they all compare unequal (except with themselves), and their hash value is derived from their id().