英文:
How does this code manage to add new elements to a set?
问题
他们首先初始化了名为rows的列表：
rows = [set() for _ in range(9)]
这意味着它创建了一个 [set(), ..., set()] 格式的列表。但后来在代码中，他们通过以下方式向这些集合添加元素：
rows[i].add(num)
从我的理解来看，集合是不可变的，那么它是如何能够以这种方式持续更新的呢？
英文:
I was working on a leetcode problem and one of the answers confused me on how it works. They first initialize the list, rows:
rows = [set() for _ in range(9)]
which I understand as creating a list of [set(), ..., set()] format. But later in the code they add to these sets by calling:
rows[i].add(num)
From my understanding, sets are immutable so how is that the set is able to be continuously updated in such a manner?
答案1
得分: 0
set不是不可变的。set的值必须是不可变的（更准确地说，是可散列的，如果它们实际上不是不可变的，则被视为不可变），但只有frozenset是真正不可变的。set被设计为可变的。
英文:
sets aren't immutable. The values of a set must be immutable (more precisely, hashable and treated as immutable if they aren't actually immutable), but only frozenset is actually immutable. sets are intended to be mutable.
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