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 that will give me same results across different platforms (Windows, Linux, Mac)?

Answer 1

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.