什么时候计算python对象的哈希值，为什么是-1的哈希值是不同的？

问题描述

继以） 。

散列值 -1 。这是因为C没有例外，所以它需要使用返回值。当一个Python对象的 __ hash __ 返回-1时，CPython实际上会默默地将其改为-2。

：

  class HashTest（object）：
 def __hash __（self）：
 print（'Yes！ __hash__被调用！'）
返回-1 
 
 hash_test = HashTest（）
 
＃所有这些都会打印出'Yes！ __hash__被调用！'：
 
 print（'__ hash__ call＃1'）
 hash_test .__ hash __（）
 
 print（'__ hash__ call＃2'）
 hash_test .__ hash __（）
 
 print（'hash call＃1'）
 hash（hash_test）
 
 print（'hash call＃2'） 
 hash（hash_test）
 
 print（'Dict creation'）
 dct = {hash_test：0} 
 
 print（'Dict get'）
 dct [hash_test] 
 
 print（'Dict set'）
 dct [hash_test] = 0 
 
 print（'__ hash__ return value：'）
 print（hash_test .__ hash __（））＃prints -1 
 print（'Actual hash value：'）
 print（hash（hash_test））＃prints -2 
  

Following on from this question, I'm interested to know when is a python object's hash computed?

1. At an instance's __init__ time,
2. The first time __hash__() is called,
3. Every time __hash__() is called, or
4. Any other opportunity I might be missing?

May this vary depending on the type of the object?

Why does hash(-1) == -2 whilst other integers are equal to their hash?

解决方案

The hash is generally computed each time it's used, as you can quite easily check yourself (see below).Of course, any particular object is free to cache its hash. For example, CPython strings do this, but tuples don't (see e.g. this rejected bug report for reasons).

The hash value -1 signalizes an error to CPython. This is because C doesn't have exceptions, so it needs to use the return value. When a Python object's __hash__ returns -1, CPython will actually silently change it to -2.

See for yourself:

class HashTest(object):
    def __hash__(self):
        print('Yes! __hash__ was called!')
        return -1

hash_test = HashTest()

# All of these will print out 'Yes! __hash__ was called!':

print('__hash__ call #1')
hash_test.__hash__()

print('__hash__ call #2')
hash_test.__hash__()

print('hash call #1')
hash(hash_test)

print('hash call #2')
hash(hash_test)

print('Dict creation')
dct = {hash_test: 0}

print('Dict get')
dct[hash_test]

print('Dict set')
dct[hash_test] = 0

print('__hash__ return value:')
print(hash_test.__hash__())  # prints -1
print('Actual hash value:')
print(hash(hash_test))  # prints -2

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