Python hash() can't handle long integer?

Question

I defined a class:

class A:
    ''' hash test class
    >>> a = A(9, 1196833379, 1, 1773396906)
    >>> hash(a)
    -340004569

    This is weird, 12544897317L expected.
    '''
    def __init__(self, a, b, c, d):
        self.a = a
        self.b = b
        self.c = c
        self.d = d

    def __hash__(self):
        return self.a * self.b + self.c * self.d

Why, in the doctest, hash() function gives a negative integer?

Answer 1

It appears to be limited to 32-bits. By reading this question, it looks like your code might have produced the expected result on a 64-bit machine (with those particular values, since the result fits in 64 bits).

The results of the built-in hash function are platform dependent and constrained to the native word size. If you need a deterministic, cross-platform hash, consider using the hashlib module.

Answer 2

See object.__hash__

Notice that

Changed in version 2.5: __hash__() may now also return a long integer object; the 32-bit integer is then derived from the hash of that object.

In your case, expected 12544897317L is a long integer object,

Python derived the 32-bit integer -340004569 by (12544897317 & 0xFFFFFFFF) - (1<<32)

Python derived the 32-bit integer by hash(12544897317L), which results -340004569

The algorithm is something like this:

def s32(x):
    x = x & ((1<<32)-1)
    if x & (1<<31):
        return x - (1<<32)
    else:
        return x

def hash(x):
    h = 0
    while x:
        h += s32(x)
        x >>= 32
    return h