问题:

I'm walking a data structure and would like to build a dict mapping X->Y, where X is a field in the data structure I'm walking and Y is a field in the data structure I'm building on the fly. X is an unhashable type.

回答1:

Trivially:

idmap = {}
idmap[id(x)] = y

Use the id of x as the dictionary key

回答2:

The purpose of Java's IdentityHashMap is to simulate dynamic field. Since Python language already supports dynamic attributes directly, you don't need the map, just assign Y to an X's attribute

x.someSuchRelation = y;

回答3:

You can just use a regular Python dict for this if you wrap your unhashable objects in another object. Specifically, something like this:

class Wrapper(object):
    def __init__(self, o):
        self.o = o

    def __hash__(self):
        return id(self.o)

    def __eq__(self, o):
        return hash(self) == hash(o)

Then just use it like some_dict[Wrapper(unhashable_object)].

This is a more useful approach than just using id(o) as the key if you also need to be able to access the object itself afterwards (as key.o, obviously). If you don't (and garbage collection isn't an issue), just use that.