Python reverse dictionary items order

Question

Assume I have a dictionary:

d = {3: 'three', 2: 'two', 1: 'one'}

I want to rearrange the order of this dictionary so that the dictionary is:

d = {1: 'one', 2: 'two', 3: 'three'}

I was thinking something like the reverse() function for lists, but that did not work. Thanks in advance for your answers!

Answer 1

Since Python 3.8 and above, the items view is iterable in reverse, so you can just do:

d = dict(reversed(d.items()))

On 3.7 and 3.6, they hadn't gotten around to implementing __reversed__ on dict and dict views (issue33462: reversible dict), so use an intermediate list or tuple, which do support reversed iteration:

d = {3: 'three', 2: 'two', 1: 'one'}
d = dict(reversed(list(d.items())))

Pre-3.6, you'd need collections.OrderedDict (both for the input and the output) to achieve the desired result. Plain dicts did not preserve any order until CPython 3.6 (as an implementation detail) and Python 3.7 (as a language guarantee).

Answer 2

Standard Python dictionaries (Before Python 3.6) don't have an order and don't guarantee order. This is exactly what the creation of OrderedDict is for.

If your Dictionary was an OrderedDict you could reverse it via:

import collections

mydict = collections.OrderedDict()
mydict['1'] = 'one'
mydict['2'] = 'two'
mydict['3'] = 'three'

collections.OrderedDict(reversed(list(mydict.items())))