Hash function: migrating from Python 2.5 to Python 2.7

[Andrin von Rechenberg]

The built-in hash function differs from Python2.5 to Python2.7.

If you are planning to migrate from Python 2.5 to Python 2.7 and

you were dumb enough to store the value of python 2.5's hash

function, just like I did, you might be interested in the following

code snippet.

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) & 0xFFFFFFFFFFFFFFFFL)[:-1])



def py25hash(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




I wonder why Google is not using 64bit hashes in Python 2.7

Cheers,
-Andrin

[Anand Mistry]

Where is this hash function coming from and what is it used for? For python27, we haven't made any changes to the interpreter/standard library to change the hash function. What you see on python.org is what you get.

[Andrin von Rechenberg]

Actually it's signed:

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) & (2**64 - 1))[:-1])

def py25hash(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

  if value > 2**63:


    value -= 2**64


  return value




On Mon, Feb 20, 2012 at 12:15 PM, Anand Mistry <ami...@google.com> wrote:

Where is this hash function coming from and what is it used for? For python27, we haven't made any changes to the interpreter/standard library to change the hash function. What you see on python.org is what you get.

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[Andrin von Rechenberg]

Ups, there was a small bug (again):

def c_mul(a, b):


  return eval(hex((long(a) * b) & (2**64 - 1))[:-1])

def py25hash(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

  if value >= 2**63:

    value -= 2**64


  return value