When is a python object's hash computed and why is the hash of -1 different?

Question

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?

Answer 1

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 signals an error in 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