Assuming that I'm using the following OrderedDict:
order_dict = OrderedDict([("a",1), ("b",2), ("c",3)])
At some point, I would like to get the (key,value) items and define an iterator, and start moving it once desired:
ordered_dict_items_iter = iter(ordered_dict.items())
...
key,val = next(ordered_dict_items_iter)
...
I'd like to know if order_dict.items() will also preserve the same order?
As I observed it seems that it does preserve the order, however I couldn't prove it.
It does. The idea of OrderedDict is that is behaves exactly as a dictionary, but internally it's a list of tuples, representing key-values pairs, so that order is preserved. All dictionary methods are replicated using this list of tuples.
Note: After python 3.7, standard dictionaries are also guaranteed to maintain insertion order.
Yes, it'll preserve the order you specify while initiliazing the dictionary.
Yes. OrderedDict.items() will return the items in the order they are inserted.
If you check the implementation of OrderedDict, you can see that items returns _OrderedDictItemsView.
class OrderedDict(dict):
...
...
def items(self):
"D.items() -> a set-like object providing a view on D's items"
return _OrderedDictItemsView(self)
And if you dig deep and find the implementation of _OrderedDictItemsView,
class _OrderedDictItemsView(_collections_abc.ItemsView):
def __reversed__(self):
for key in reversed(self._mapping):
yield (key, self._mapping[key])
And if you go deeper to check the _collections_abc.ItemsView, you will see that,
class ItemsView(MappingView, Set):
...
...
def __iter__(self):
for key in self._mapping:
yield (key, self._mapping[key])
And further down the MappingView, you will see,
class MappingView(Sized):
__slots__ = '_mapping',
def __init__(self, mapping):
self._mapping = mapping
Now our journey has reached it's destination, and we can see that the _mapping is the OrderedDict we created and it is always in order. The __iter__ method ItemsView, just iterates through each key value in the OrderedDict. Hence the proof :)
