Lists and Tuples and Much More

James Stroud jstroud at
Sun Apr 15 00:45:50 CEST 2007

Hendrik van Rooyen wrote:
>  "7stud" <bb..9_05ss at> wrote:
>>..........  But using a tuple as a
>>key in a dictionary is probably something you will never do.
> Yikes!  I do this all the time...
> Think of an address in any one town.
> It has a Street, and a number 
> (could be more complex, like a sub number for an apartment 
> in a block, but lets ignore that for now)
> so:
> addy = ("First Avenue", 10)
> Residents = {addy:"Frank Everyman",......}
> To make a thing where you can look up who lives at a place...
> (this is simplistic of course, for illustrative purposes - few people
> live completely alone. - it should be at least a list, or possibly a dict 
> by surname...)
> I find that when I do this, I almost invariably also create the
> inverse dict:
> Addresses = {"Frank Everyman":addy,.....}
> So that you can get at stuff from all directions.  If you keep it
> up you have a home made relational database if you are not careful...
> But if you limit it to one thing and its inverse, its quite useful, and it
> would be nice to have one "doubledict" that can be accessed as speedily
> from either end...
> Sort of an internally linked list of mixed hashed immutables, where: 
> doubledict["Frank Everyman"] yields addy, and
> doubledict[addy] yields "Frank Everyman"
> It would have direct applicability in things like the label table in
> an assembler, for debugging and so on.
> - Hendrik

I'm thinking that, to avoid big a lot of ambiguity, such a double dict 
would need to be implemented as two distinctly referenced data structures:

class DD(object):
   def __init__(self, adict):
     self._fwd = {}
     self._bkwk = dict(v,k for k,v in adict.items())
   def fwd(self, k)
     return self._fwd[k]
   def bkwd(self, k)
     return self._bkwd[k]
   def __setitem__(self, k, v):
     self._fwd[k] = v
     self._bkwd[v] = k
   def __getitem__(self, k):
     if (k in self._fwd) and (k in self._bkwd):
       raise KeyError, 'Which do I look in first?'
       raise KeyError, 'Too much guesswork. Use fwd() or bkwd().'


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