Storing 'unhashable' types in dictionaries by address

[Ed Avis]

A dictionary requires that its key type be hashable.  This means you
cannot use lists as keys of a dictionary.  Is there any way round
this?

I would like to store some 'extra' information associated with some
lists, but do so outside the lists themselves.  For example this is
what I would like to write:

l0 = ['strawberry']
l1 = ['mint', 'choc']
l2 = ['vanilla', 'liver']

# This dictionary stores some extra comments on each flavour.
d = {}
d[l0] = 'yum'
d[l1] = 'so-so'
d[l2] = 'ugh'

print d[l0]    # prints 'yum'
l0.append(55)
print d[l0]    # still prints 'yum', since it's the same list

# Now I construct a new list which, although it contains the same
# strings as l1, is a different object.  This is just to demonstrate
# that d indexes by 'is' rather than by ==.
#
new_list = ['mint', 'choc']
print new_list in d.keys()    # prints 0, or false


I hope the intent is clear - I want to look up by object identity and
not by the contents of the list.

Something which would be almost as good is to take the address of a
list or other object, then I could use that as the key of a
dictionary.

Is there any library which can do what I want, either by providing its
own dictionaries which don't require objects to be hashable, or by
letting the programmer get the address of an object?

-- 
Ed Avis <ed at membled.com>