Dictionary keys (again) (was Re: lambda)

[Steven Bethard]

Nick Coghlan wrote:
> For a 'mutable key' to make sense, the following:
> 
> lst = []
> dct = {l: "Hi!"}
> print dct[[]]
> print dct[lst]
> lst.append(1)
> print dct[[1]]
> print dct[lst]
> 
> Should print:
> Hi
> Hi
> Hi
> Hi

And here's an implementation that does so:

py> class sillydict(dict):
...     def __getitem__(self, key):
...         items = self.items()
...         self.clear()
...         self.update(items)
...         return super(sillydict, self).__getitem__(key)
...
py> class sillylist(list):
...     def __hash__(self):
...         return hash(tuple(self))
...
py> lst = sillylist([])
py> dct = sillydict({lst: "Hi!"})
py> print dct[sillylist([])]
Hi!
py> print dct[lst]
Hi!
py> lst.append(1)
py> print dct[sillylist([1])]
Hi!
py> print dct[lst]
Hi!

Of course, as noted by the classes themselves, they're silly. ;)  But as 
long as you don't mind rehashing the dict each time the dict is accessed 
;) you can get sane behavior even in the presence of mutable keys.

Steve