Objects with user-defined hash

Question

Assume I have a class A with a overriden hash method that returns some user-defined integer:

class A:
   def __init__(self,hash):
      self.hash = hash

   def __hash__(self):
      return self.hash

   def __cmp__(self,other):
      return cmp(self.hash,other.hash)

Now, at any given point in time, I'd like to have only one object with the same hash, so I maintain a set s that contains such objects of class A. My problem is the following:

s = {A(1234)} 
a = A(1234)

if a in s:
   # then assign the corresponding object in set s to a

How can I achieve this?

Thanks!

Answer 1

Don't use a set, use a dictionary (which is also a set, in a sense).

objects = {}
a = A(1234)
if a.hash in objects:
    a = objects[a.hash]
objects[a.hash] = a

Answer 2

I'd use a singleton implemented as a class variable:

>>> class A:
    HASH = 0
    def __init__(self):
        self.hash = A.HASH
        A.HASH += 1
    def __hash__(self):
        return self.hash
    def __cmp__(self,other):
        return cmp(self.hash, other.hash)


>>> a = A()
>>> a.__hash__()
0
>>> a2 = A()
>>> a2.__hash__()
1
>>> 

Since it's increased each time you instanciate a new object, you're sure not to have twice the same value (this might not be thread-safe, though).

EDIT : if the hash value is computed, this solution is not valid since it starts from 0 arbitrarily...

Answer 3

I used the following mechanism to make sure that no duplicate object was ever created. This is a mixture of Emmanuel's and Jordan's answers.

class A(object):
   __singletons__ = dict()

   def __new__(cls,hash):
      if hash not in cls.__singletons__.keys():
         cls.__singletons__[hash] = super(A,cls).__new__(cls)

      return cls.__singletons__[hash]

   def __init__(self,hash):
      self.hash = hash

   def __hash__(self):
      return self.hash

   def __cmp__(self,other):
      return cmp(self.hash,other.hash)