When is hash(n) == n in Python?
I've been playing with Python's hash function. For small integers, it appears
hash(n) == n always. However this does not extend to large numbers:
>>> hash(2**100) == 2**100 False
I'm not surprised, I understand hash takes a finite range of values. What is that range?
I tried using binary search to find the smallest number
hash(n) != n
>>> import codejamhelpers # pip install codejamhelpers >>> help(codejamhelpers.binary_search) Help on function binary_search in module codejamhelpers.binary_search: binary_search(f, t) Given an increasing function :math:`f`, find the greatest non-negative integer :math:`n` such that :math:`f(n) \le t`. If :math:`f(n) > t` for all :math:`n \ge 0`, return None. >>> f = lambda n: int(hash(n) != n) >>> n = codejamhelpers.binary_search(f, 0) >>> hash(n) 2305843009213693950 >>> hash(n+1) 0
What's special about 2305843009213693951? I note it's less than
sys.maxsize == 9223372036854775807
Edit: I'm using Python 3. I ran the same binary search on Python 2 and got a different result 2147483648, which I note is
I also played with
[hash(random.random()) for i in range(10**6)] to estimate the range of hash function. The max is consistently below n above. Comparing the min, it seems Python 3's hash is always positively valued, whereas Python 2's hash can take negative values.
Based on python documentation in
For numeric types, the hash of a number x is based on the reduction of x modulo the prime
P = 2**_PyHASH_BITS - 1. It's designed so that
hash(x) == hash(y)whenever x and y are numerically equal, even if x and y have different types.
So for a 64/32 bit machine, the reduction would be 2 _PyHASH_BITS - 1, but what is
You can find it in
pyhash.h header file which for a 64 bit machine has been defined as 61 (you can read more explanation in
#if SIZEOF_VOID_P >= 8 # define _PyHASH_BITS 61 #else # define _PyHASH_BITS 31 #endif
So first off all it's based on your platform for example in my 64bit Linux platform the reduction is 261-1, which is
>>> 2**61 - 1 2305843009213693951
Also You can use
math.frexp in order to get the mantissa and exponent of
sys.maxint which for a 64 bit machine shows that max int is 263:
>>> import math >>> math.frexp(sys.maxint) (0.5, 64)
And you can see the difference by a simple test:
>>> hash(2**62) == 2**62 True >>> hash(2**63) == 2**63 False
Read the complete documentation about python hashing algorithm https://github.com/python/cpython/blob/master/Python/pyhash.c#L34
As mentioned in comment you can use
sys.hash_info (in python 3.X) which will give you a struct sequence of parameters used for computing hashes.
>>> sys.hash_info sys.hash_info(width=64, modulus=2305843009213693951, inf=314159, nan=0, imag=1000003, algorithm='siphash24', hash_bits=64, seed_bits=128, cutoff=0) >>>
Alongside the modulus that I've described in preceding lines, you can also get the
inf value as following:
>>> hash(float('inf')) 314159 >>> sys.hash_info.inf 314159
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