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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)?
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.
Use hashlib as hash() was designed to be used to:
quickly compare dictionary keys during a dictionary lookup
and therefore does not guarantee that it will be the same across Python implementations.
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.
