When is hash(n) == n in Python?

2305843009213693951 is 2^61 - 1. It's the largest Mersenne prime that fits into 64 bits.

If you have to make a hash just by taking the value mod some number, then a large Mersenne prime is a good choice -- it's easy to compute and ensures an even distribution of possibilities. (Although I personally would never make a hash this way)

It's especially convenient to compute the modulus for floating point numbers. They have an exponential component that multiplies the whole number by 2^x. Since 2^61 = 1 mod 2^61-1, you only need to consider the (exponent) mod 61.

See: https://en.wikipedia.org/wiki/Mersenne_prime

Based on python documentation in pyhash.c file:

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 _PyHASH_BITS?

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 pyconfig.h file).

#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 2305843009213693951:

>>> 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

Hash function returns plain int that means that returned value is greater than -sys.maxint and lower than sys.maxint, which means if you pass sys.maxint + x to it result would be -sys.maxint + (x - 2).

hash(sys.maxint + 1) == sys.maxint + 1 # False
hash(sys.maxint + 1) == - sys.maxint -1 # True
hash(sys.maxint + sys.maxint) == -sys.maxint + sys.maxint - 2 # True

Meanwhile 2**200 is a n times greater than sys.maxint - my guess is that hash would go over range -sys.maxint..+sys.maxint n times until it stops on plain integer in that range, like in code snippets above..

So generally, for any n <= sys.maxint:

hash(sys.maxint*n) == -sys.maxint*(n%2) +  2*(n%2)*sys.maxint - n/2 - (n + 1)%2 ## True

Note: this is true for python 2.