How to get the "next" item in an OrderedDict?

Question

I'm using an OrderedDict to random access a list, but now want the next item in the list from the one that I have:

foo = OrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)])
apple = foo['apple']

How do I get the banana using just foo and apple?

Answer 1

If you are OK with accessing those parts of the OrderedDict implementation that are intentionally kept private:

>>> class MyOrderedDict(OrderedDict):
...     def next_key(self, key):
...             next = self._OrderedDict__map[key][1]
...             if next is self._OrderedDict__root:
...                     raise ValueError("{!r} is the last key".format(key))
...             return next[2]
...     def first_key(self):
...             for key in self: return key
...             raise ValueError("OrderedDict() is empty")
... 
>>> od = MyOrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)])
>>> od.next_key("apple")
'banana'
>>> od.next_key("banana")
'orange'
>>> od.next_key("orange")
'pear'
>>> od.next_key("pear")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in next_key
ValueError: 'pear' is the last key
>>> od.first_key()
'apple'