python bijection

[Gabriel Genellina]

En Sat, 28 Nov 2009 06:30:44 -0300, Joshua Bronson <jabronson at gmail.com>  
escribió:
> On Nov 27, 9:36 pm, "Gabriel Genellina" <gagsl-py2 at yahoo.com.ar>
> wrote:
>> En Fri, 27 Nov 2009 15:12:36 -0300, Francis Carr  
>> <coldtortuga at gmail.com> escribió:
>>
>> > After much tinkering, I think I have a simpler solution.  Just make
>> > the inverse mapping accessible via an attribute, -AND- bind the
>> > inverse of -THAT- mapping back to the original.  The result is a
>> > python dict with NO NEW METHODS except this inverse-mapping
>> > attribute.  I have posted it on code.activestate.com as <a
>> > href="http://code.activestate.com/recipes/576968/">Recipe 576968:
>> > Flipdict -- python dict that also maintains a one-to-one inverse
>> > mapping</a>
>
>> Just a couple of comments:
>>
>> Instead of:
>>         self._flip = dict.__new__(self.__class__)
>> I'd write:
>>         self._flip = self.__class__()
>> unless I'm missing something (but see the next point).
>
> How would this not cause infinite recursion?

That goes under "unless I'm missing something" ;)
You're right, it would cause infinite recursion. Not a good idea...

>> Also, although Python's GC is able to handle them, I prefer to avoid  
>> circular references like those between x and x._flip.  Making  
>> self._flip a weak reference (and dereferencing it in the property)  
>> should be enough.
>
> If both self._flip and self._flip._flip are weak references, no strong
> references to the inverse mapping survive leaving the constructor
> scope. Unless I'm missing something, only one of these can be a weak
> reference, and then you'd have to do something like this in the
> property to prevent "TypeError: FlipDict is not callable":
>
>     @property
>     def flip(self):
>         try:
>             # we're an inverse, self._flip is a weak reference
>             return self._flip()
>         except TypeError:
>             # we're a forward mapping, self._flip is a strong
> reference
>             return self._flip

Yes - although I'd explicitely test for a weakref object:

     def flip(self):
         _flip = self._flip
         if isinstance(_filp, weakref.ref):
             return _flip()
         return _flip

and probably all those internal references to self._flip should become  
self.flip too; I've not tested the code but I think it *should* work...

-- 
Gabriel Genellina