用户定义类的默认哈希值是多少？

Question

docs 错误地声称

Objects which are instances of user-defined classes are hashable by default; they all compare unequal (except with themselves), and their hash value is their id()

虽然我记得这曾经是正确的，但在 python（v2.7.10、v3.5.0）的当前版本中，此类散列等于其 id 的对象显然不正确。

>>> class A:
...     pass
... 
>>> a = A()
>>> hash(a)
-9223372036578022804
>>> id(a)
4428048072

在文档的 another part 中说哈希是 从 id 派生的 。 When/why实现是否改变，hash返回的数字"derived from"现在的id如何？

Answer 1

相关函数似乎是：

Py_hash_t
_Py_HashPointer(void *p)
{
    Py_hash_t x;
    size_t y = (size_t)p;
    /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid
       excessive hash collisions for dicts and sets */
    y = (y >> 4) | (y << (8 * SIZEOF_VOID_P - 4));
    x = (Py_hash_t)y;
    if (x == -1)
        x = -2;
    return x;
}

（该代码来自 here, and is then used to be the tp_hash slot in type here.) The comment there seems to give a reason for not using the pointer (which is the same thing as the id) directly. Indeed, the commit that introduced that change to the function is here，并指出更改的原因是：

Issue #5186: Reduce hash collisions for objects with no hash method by rotating the object pointer by 4 bits to the right.

指的是 this 问题，该问题更多地解释了进行更改的原因。

Answer 2

由于 issue #5186，这在 2009 年发生了变化；通常的 id() 值导致了太多的冲突：

In the issue 5169 discussion, Antoine Pitrou suggested that for an object 
x without a `__hash__` method, `id()/8` might be a better hash value than 
`id()`, since dicts use the low order bits of the hash as initial key, and 
the 3 lowest bits of an `id()` will always be zero.

current implementation 获取 id 并旋转它以产生更多变化的值：

long
_Py_HashPointer(void *p)
{
    long x;
    size_t y = (size_t)p;
    /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid
       excessive hash collisions for dicts and sets */
    y = (y >> 4) | (y << (8 * SIZEOF_VOID_P - 4));
    x = (long)y;
    if (x == -1)
        x = -2;
    return x;
}

这导致 14% 到 34% 的加速，具体取决于执行的测试。

词汇表已经过时了；我看到你了already opened an issue with the project。

用户定义类的默认哈希值是多少？

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用户定义 类 的默认哈希值是多少？

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用户定义类的默认哈希值是多少？