Finding the type of indexing supported by an object?

[Derek Peschel]

Here are two functions.

def invert_dict_to_lists(dict):
    lists = {}
    for key in dict:
        value = dict[key]
        if not value in lists:
            lists[value] = [key]
        else:
            lists[value].append(key)
    return lists

def invert_list_to_lists(list):
    lists = {}
    for key in range(len(list)):
        value = list[key]
        if not value in lists:
             lists[value] = [key]
        else:
            lists[value].append(key)
    return lists

They are the same except for the expression in "for key in ...".  Can they
be combined into one function?  How can I determine if the argument is
like a list (with numeric indices that are not stored in the list) or a dict
(with arbitrary keys that are stored)?  I said "object" in the subject,
but I want to support Python primitive types, class instances, extension
module types (array, dictproxy, dbm, gdbm, etc.), and any future types.

I've thought about looking for keys(), looking for the special method names
that allow you to override indexing behavior, and looking at the class or
type of the object.  I could be wrong, but I don't think any of those
strategies will work with all arguments.

Thanks,

-- Derek