Built in python hash() function

Question

Windows XP, Python 2.5:

hash('http://stackoverflow.com') Result: 1934711907

Google App Engine (http://shell.appspot.com/):

hash('http://stackoverflow.com') Result: -5768830964305142685

Why is that? How can I have a hash function which will give me same results across different platforms (Windows, Linux, Mac)?

Answer 1

use hashlib

Answer 2

It probably just asks the operating system provided function, rather than its own algorithm.

As other comments says, use hashlib or write your own hash function.

Answer 3

As stated in the documentation, built-in hash() function is not designed for storing resulting hashes somewhere externally. It is used to provide object's hash value, to store them in dictionaries and so on. It's also implementation-specific (GAE uses a modified version of Python). Check out:

>>> class Foo:
...     pass
... 
>>> a = Foo()
>>> b = Foo()
>>> hash(a), hash(b)
(-1210747828, -1210747892)

As you can see, they are different, as hash() uses object's hash method instead of 'normal' hashing algorithms, such as SHA.

Given the above, the rational choice is to use the hashlib module.

Answer 4

At a guess, AppEngine is using a 64-bit implementation of Python (-5768830964305142685 won't fit in 32 bits) and your implementation of Python is 32 bits. You can't rely on object hashes being meaningfully comparable between different implementations.

Answer 5

The response is absolutely no surprise: in fact

In [1]: -5768830964305142685L & 0xffffffff
Out[1]: 1934711907L

so if you want to get reliable responses on ASCII strings, just get the lower 32 bits as uint. The hash function for strings is 32-bit-safe and almost portable.

On the other side, you can't rely at all on getting the hash() of any object over which you haven't explicitly defined the __hash__ method to be invariant.

Over ASCII strings it works just because the hash is calculated on the single characters forming the string, like the following:

class string:
    def __hash__(self):
        if not self:
            return 0 # empty
        value = ord(self[0]) << 7
        for char in self:
            value = c_mul(1000003, value) ^ ord(char)
        value = value ^ len(self)
        if value == -1:
            value = -2
        return value

where the c_mul function is the "cyclic" multiplication (without overflow) as in C.