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

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Hash results varies between 32bit and 64bit platforms  If a calculated hash shall be the same on both platforms consider using  def hash32 value       return hash value   amp  0xffffffff

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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

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Polynomial hash for strings  1000000009 and 239 are arbitrary prime numbers  Unlikely to have collisions by accident  Modular arithmetic is not very fast  but for preventing collisions this is more reliable than taking it modulo a power of 2  Of course  it is easy to find a collision on purpose   mod 1000000009 def hash s       result 0     for c in s          result    result   239   ord c     mod     return result   mod

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Most answers suggest this is because of different platforms  but there is more to it  From the documentation of object   hash   self       By default  the   hash     values of str  bytes and   datetime objects are    salted    with an unpredictable random value    Although they remain constant within an individual Python process    they are not predictable between repeated invocations of Python       This is intended to provide protection against a denial-of-service   caused by carefully-chosen inputs that exploit the worst case   performance of a dict insertion  O n    complexity  See   http   www ocert org advisories ocert-2011-003 html for details       Changing hash values affects the iteration order of dicts  sets   and other mappings  Python has never made guarantees about this   ordering  and it typically varies between 32-bit and 64-bit builds     Even running on the same machine will yield varying results across invocations     python -c  print hash  http   stackoverflow com     -3455286212422042986   python -c  print hash  http   stackoverflow com     -6940441840934557333   While     python -c  print hash  1 2 3     2528502973977326415   python -c  print hash  1 2 3     2528502973977326415     See also the environment variable PYTHONHASHSEED      If this variable is not set or set to random  a random value is used   to seed the hashes of str  bytes and datetime objects       If PYTHONHASHSEED is set to an integer value  it is used as a fixed   seed for generating the hash   of the types covered by the hash   randomization       Its purpose is to allow repeatable hashing  such as for selftests for   the interpreter itself  or to allow a cluster of python processes to   share hash values       The integer must be a decimal number in the range  0  4294967295     Specifying the value 0 will disable hash randomization    For example     export PYTHONHASHSEED 0                               python -c  print hash  http   stackoverflow com     -5843046192888932305   python -c  print hash  http   stackoverflow com     -5843046192888932305

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This is the hash function that Google uses in production for python 2 5   def c mul a  b     return eval hex  long a    b   amp   2  64 - 1    -1    def py25hash self     if not self      return 0   empty   value   ord self 0    lt  lt  7   for char in self      value   c mul 1000003  value    ord char    value   value   len self    if value    -1      value   -2   if value  gt   2  63      value -  2  64   return value

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The response is absolutely no surprise  in fact   In  1   -5768830964305142685L  amp  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    lt  lt  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

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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

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What about sign bit   For example   Hex value 0xADFE74A5 represents unsigned 2919134373 and signed -1375832923  Currect value must be signed  sign bit   1  but python converts it as unsigned and we have an incorrect hash value after translation from 64 to 32 bit   Be careful using   def hash32 value       return hash value   amp  0xffffffff

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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    gt  gt  gt  class Foo          pass       gt  gt  gt  a   Foo    gt  gt  gt  b   Foo    gt  gt  gt  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

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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

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The value of PYTHONHASHSEED might be used to initialize the hash values   Try   PYTHONHASHSEED python -c  print hash  http   stackoverflow com

[python] Built in Python hash() function

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