Looping OrderedDict in reverse order

[mishal]

I have an OrderedList of items and I would like to loop the items in reverse order like:

from collections import OrderedDict as TaskDict

d = TaskDict()
d["a"] = "something A"
d["c"] = "something C"
d["b"] = "something B"

print("Normal:")
for k, v in d.items():
    print(f"k: {k}, v: {v}")

print("Reversed:")
# reversed loop
for k, v in reversed(d.items()):
    print(f"k: {k}, v: {v}")

This produces following running on micropython (1.20.0, linux build):

Normal:
k: a, v: something A
k: c, v: something C
k: b, v: something B
Reversed:
Traceback (most recent call last):
  File "./tests/test_task_dict.py", line 15, in <module>
TypeError: object of type 'dict_view' has no len()

The code works fine using python 3.9.2 /3.5.2/, so the expected output should be:

Normal:
k: a, v: something A
k: c, v: something C
k: b, v: something B
Reversed:
k: b, v: something B
k: c, v: something C
k: a, v: something A

Looking at python's 3.5.2 docs. The docs here: https://docs.python.org/3.5/library/collections.html#collections.OrderedDict say that the object must have either __reversed__ method or support sequence protocol... see https://docs.python.org/3.5/library/functions.html#reversed

Is this a bug or am I missing something? Thanks

[mishal]

According to docs https://docs.micropython.org/en/latest/genrst/builtin_types.html?highlight=dict_view#dict which propose a workaround to explicitly convert keys to a set before using set operations.

I converted them to a list which can be reverted using:

for k in reversed(list(d.keys())):
    v = d[k]
    print(f"k: {k}, v: {v}")

[jimmo]

@mishal Yep, this is the best workaround.

[sosi-deadeye]

Micropython is developed for Microcontrollers and must be very memory efficient. The reversed method is not implemented for OrderedDict nor for dict.

Possible inefficient solution:

from collections import OrderedDict


class ItemsView:
    """
    ItemsView to support reversed
    But this implementation is memory-inefficient
    """
    def __init__(self, parent):
        self.parent = parent
    
    def __reversed__(self):
        for key in reversed(list(self.parent)):
            yield key, self.parent[key]


class TaskDict(OrderedDict):
    def items(self):
        return ItemsView(self)


print(list(reversed(TaskDict(a=10, b=20, c=30).items())))

It's simpler to use a list for tasks instead of a dict.
Or use collections.deque. It has the methods append and popleft.

[mishal]

Thanks @sosi-deadeye .

I need a dictionary, so for now I will just use this, I will try to implement my code using lists when optimizing the app.

Also to note is that micropython's dict method keys() does not return the keys in the order they were added, this is python 3.7+ language specification - see https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects