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pythonhashtypes
Seungho Lee
Seungho Lee

Reputation: 1155

Why does hash() method return short Hash value with int in Python?

When hash() method is called in Python 3, I noticed that it doesn't return a long-length integer when taking in int data type but with string type.

Is this supposed to work this way? If that actually is the case, for the int type to have a short hash value, won't it cause collision since it's too short?

for i in [i for i in range(5)]:
    print(hash(i))

print(hash("abc"))

The Result:

0
1
2
3
4
4714025963994714141

Upvotes: 0

Views: 3758

Answers (3)

ingvar
ingvar

Reputation: 4377

In CPython, default Python interpreter implementation, built-in hash is done in this way:

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

_PyHASH_BITS is 61 (64-bit systems) or 31 (32-bit systems)(defined here)

So on 64-bit system built-in hash looks like this function:

def hash(number):
    return number % (2 ** 61 - 1)

That's why for small ints you got the same values, while for example hash(2305843009213693950) returns 2305843009213693950 and hash(2305843009213693951) returns 0

Upvotes: 9

Sdrf1445
Sdrf1445

Reputation: 147

You should use hashlib module:

>>> import hashlib()
>>> m.update(b'abc')
>>> m.hexdigest()

Upvotes: 0

chepner
chepner

Reputation: 530823

The only purpose of the hash function is to produce an integer value that can be used to insert an object into a dict. The only thing hash guarantees is that if a == b, then hash(a) == hash(b). For a user-defined class Foo, it is the user's responsibility to ensure that Foo.__eq__ and Foo.__hash__ enforce this guarantee.

Anything else is implementation-dependent, and you shouldn't read anything into the value of hash(x) for any value x. Specifically, hash(a) == hash(b) is allowed for a != b, and hash(x) == x is not required for any particular x.

Upvotes: 4

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