# Re: Dictionary, keys and alias

I think "hash" doesn't guarantee the unicity of the result. But, it should avoid the collisions...

>>> foo = "foo"
>>> hash(foo)
-740391237
>>> hash(-740391237)
-740391237

I think it's like some kind md5sum...

I propose this solution:

-----------------------------------------------------------
from UserDict import UserDict

class DoubleDict(UserDict):
def __init__(self, *args):
UserDict.__init__(self, *args)
self.values = {}
#self.aliases = {}

def setAlias(self, key, alias):
# Point the alias to the same hash as the real key.
if key in self:
self.data[alias] = self.data[key]
else:
self[alias] = key

def __getitem__(self, key):
return self.values[self.data[key]]

def __setitem__(self, key, value):
self.data.setdefault(key, key)
self.values[self.data[key]] = value

print "Result:"
d = DoubleDict()
d["a"] = 1
d["c"] = 4
d.setAlias("a", "b")
print d["a"], d["b"], d["c"]
d["a"] = 2
print d["a"], d["b"], d["c"]
d["b"] = 3
d.setAlias("b", "c")
print d["a"], d["b"], d["c"]
del d["c"]
d["c"] = 5
print d["a"], d["b"], d["c"]
del d["a"]
d["a"] = 6
print d["a"], d["b"], d["c"]

-------------------------------------------------------
Result:
1 1 4
2 2 4
3 3 3
3 3 5
6 6 5

As you can see the last test (del "a" and reassign "a") fail because it reassign "b".
But, if your application doesn't need to midify the aliases, it's ok (I think). Else you can redefine the method __delitem__.

Cyril

On 7/18/05, Steven D'Aprano <[EMAIL PROTECTED]> wrote:
On Mon, 18 Jul 2005 12:17:37 +0200, Glauco wrote:

> I want to insert a concept of alias in a dict_based class.
>
> The idea  is to have a facoltative name in the same dict that correspond
> at the same value. With this alias i can change original value.
>
> example:
>
> mydict['a'] = 1
> I must define an alias  example: myFunctAlias( mydict, 'a', 'b')
> print mydict
> {'a':1, 'b':1}
> mydict['b'] = 2
> print mydict
> {'a':2, 'b':2}
>
>
> The only idea i have is to implement two dictionary one for convert
> name, alias in two keys with the same value (eg.numeric) in the first
> dict. The second for store only one time the k, v .

You need some sort of redirection, something like this (untested):

class Doubledict:
def __init__(self, **kargs):
self.data = "">        self.aliases = {}
for key in kargs:
# Point the key to a hash.
self.aliases[key] = hash(key)
# And point the hash at the value.
self.data[hash(key)] = kargs[key]

def setalias(self, key, alias):
# Point the alias to the same hash as the real key.
self.aliases[alias] = hash(key)

def __getitem__(self, key):
return self.data[self.aliases[key]]

def __setitem__(self, key, value):
self.data[self.aliases[key]] = value

The only niggly worry I have is I'm not sure when hash can be used, when
it is unique, or even if is it guaranteed to be unique.

--
Steven.

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