How to lazily iterate on reverse order of keys in OrderedDict?

Question

I'd like to iterate on an OrderedDict in reverse order.

I.e. reverse the order of:

for k, v in my_ordered_dict.iteritems():
   # < do stuff >

So far I've gotten a non-lazy version, by reversing a list:

for k, v in list(my_ordered_dict.iteritems())[::-1]:
   # < do stuff >

Any ideas how to make it better?

`reverse()` isn't lazy - it needs to evaluate the whole `iterable` to return it in reverse order, `for k, v in reversed(my_ordered_dict.iteritems())` is about the best you can do. — AChampion, Mar 20 '17 at 05:53
@AChampion, in that case `reversed(my_ordered_dict.iteritems())` is equivalent to `list(my_ordered_dict.iteritems())[::-1]`? — Yuval Atzmon, Mar 20 '17 at 07:45
No, this creates 2 lists, one for `list()` and one for `[::-1]` vs. just one for `reversed(...)` — AChampion, Mar 20 '17 at 12:10
@AChampion, I think you are mistaken. It looks like OrderedDict has the `__reversed__` method implemented — Yuval Atzmon, Mar 20 '17 at 12:23

score 14 · Accepted Answer · edited Mar 20 '17 at 18:25

14

If you use reversed on the dict it should evaluate lazily:

for k, v in ((k, my_ordered_dict[k]) for k in reversed(my_ordered_dict)):
    .....

How does this work?

The key element here is a generator expression, which is evaluated lazily. So this will lazily evaluate over the keys of the ordered dict, and then return a tuple of the key and the dict value when needed.

I only need to support python 3:

In python 3, dict.items() is now a view and is evaluated lazily. Thus the above can be simplified to:

for k, v in reversed(my_ordered_dict.items()):

While Python 2.7 does have a viewitems() method, Python 2.7 OrderedDict views do not support the __reversed__ hook that reversed needs to reverse non-sequences.

edited Mar 20 '17 at 18:25

user2357112

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answered Mar 20 '17 at 05:02

Stephen Rauch

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could you simplify this further to `for k, v in reversed(my_ordered_dict.items()):` ? – maxymoo Mar 20 '17 at 05:13
And why wouldn't `for k, v in reversed(my_ordered_dict.iteritems())` work for Py2? – AChampion Mar 20 '17 at 05:52
@AChampion: `reversed` doesn't accept iterators. (`viewitems` won't work either, because Python 2 OrderedDict views don't support `__reversed__`.) – user2357112 Mar 20 '17 at 18:23
Good point. Then `for k in reversed(my_ordered_dict): v = my_ordered_dict[k] ...`, as `OrderedDict` does have `__reversed__()` Py2. – AChampion Mar 20 '17 at 20:55

score -2 · Answer 2 · answered Mar 20 '17 at 05:03

-2

You could for in range with the values and then subtract from the number of values.

items = my_ordered_dict.iteritems()
max_index = len(items)

for i in range(max_index):
    object = items[max_index - i - 1]

answered Mar 20 '17 at 05:03

meyer9

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In Python 2.7—which it looks like you're using—`max_index = len(items)` -> `TypeError: object of type 'generator' has no len()`. In Python 3, using `my_ordered_dict.items()` results in `TypeError: 'odict_items' object does not support indexing` for the line `object = items[max_index - i - 1]`. – martineau Dec 31 '18 at 01:22

How to *lazily* iterate on reverse order of keys in OrderedDict?

2 Answers2

How to lazily iterate on reverse order of keys in OrderedDict?