Lookup table for unhashable in Python

Question

I need to create a mapping from objects of my own custom class (derived from dict) to objects of another custom class. As I see it there are two ways of doing this:

1. I can make the objects hashable. I'm not sure how I would do this. I know I can implement __hash__() but I'm unsure how to actually calculate the hash (which should be an integer).

2. Since my objects can be compared I can make a list [(myobj, myotherobj)] and then implement a lookup which finds the tuple where the first item in the tuple is the same as the lookup key. Implementing this is trivial (the number of objects is small) but I want to avoid reinventing the wheel if something like this already exists in the standard library.

It seems to me that wanting to look up unhashables would be a common problem so I assume someone has already solved this problem. Any suggestions on how to implement __hash()__ for a dict-like object or if there is some other standard way of making lookup tables of unhashables?

Answer 1

Mappings with mutable objects as keys are generally difficult. Is that really what you want? If you consider your objects to be immutable (there is no way to really enforce immutability in Python), or you know they will not be changed while they are used as keys in a mapping, you can implement your own hash-function for them in several ways. For instance, if your object only has hashable data-members, you can return the hash of a tuple of all data-members as the objects hash.

If your object is a dict-like, you can use the hash of a frozenset of all key-value-pairs.

def __hash__(self):
    return hash(frozenset(self.iteritems()))

This only works if all values are hashable. In order to save recalculations of the hashes (which would be done on every lookup), you can cache the hash-value and just recalculate it if some dirty-flag is set.

Answer 2

A simple solution seems to be to do lookup[id(myobj)] = myotherobj instead of lookup[myobj] = myotherobj. Any commente on this approach?