Order of keys in dictionary

Question

I have a dictionary

{'a': 'first', 'b': 'second'}

However, I need the dictionary in a different order:

{'b': 'second', 'a': 'first'}

What is the best way to do this?

Mulish

Ok, my fault, I'll read documentation. I just print it and it was out by specific order by chance. I thought it has something to do with a way it is written. Like php's array. – Mulish

@Mulish This is actually a good question. I was like "oh, that'll be easy to answer with the documentation" -- it isn't. This "trivial fact" seems to be assumed knowledge in most places. The best I can find (in OrderedDict, chapter 8.3) is "An OrderedDict is a dict that remembers the order that keys were first inserted.". – Trinity

Ahh, there it is: "It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary)." from 5.5 Data Structures in the Tutorial. – Trinity

-1. Searching StackOverflow for "python dictionary order" easily finds the answer. You should at least try something before posting to StackOverflow. – Sinasinai

possible duplicate of In what order does python display dictionary keys? – Antihalation

@pst: Also Library Reference, Ch. 5 "Built-in Types": <quote>CPython implementation detail: Keys and values are listed in an arbitrary order which is non-random, varies across Python implementations, and depends on the dictionary’s history of insertions and deletions.</quote> – Amicable

@Steven Rumbalski, I was looking for "how to change dictionary order?", not for "is it possible to change dictionary order?". They are different questions, though have the same answer. – Mulish

@Qiao: If they have the same answer, then they're dupes. The idea is to get all the answers in one place. – Amicable

possible duplicate of How to reverse order of keys in python dict? – Postglacial

Answer 1

Dictionaries are not ordered. So there is no way to do it.

If you have python2.7+, you can use collections.OrderedDict - in this case you could retrieve the item list using .items() and then reverse it and create a new OrderedDict from the reversed list:

>>> od = OrderedDict((('a', 'first'), ('b', 'second')))
>>> od
OrderedDict([('a', 'first'), ('b', 'second')])
>>> items = od.items()  # list(od.items()) in Python3
>>> items.reverse()
>>> OrderedDict(items)
OrderedDict([('b', 'second'), ('a', 'first')])

If you are using an older python version you can get a backport from http://code.activestate.com/recipes/576693/

Linkous

Exactly, both dictionaries in your example are equivalent. – Balakirev

Sorted and ordered are different thing. OrderedDict isn't sorted, can't be sorted manually and no data structure in Python is a sorted data structure (i.e. always automatically sorted). – Rosie

That's what I meant. Edited my answer for clarity though. – Linkous

@Roxy Oxymoron But an OrderedDict can be created from a sorted list. The question isn't: "how can I created a dict that is always sorted?" – Trinity

Does not work in Python 3.4. The line items = od.items() returns an ItemsView object, which does not have a reverse() function. – Diviner

Updated my answer to mention how to do it in Python 3. – Linkous

Answer 2

Dictionaries don't have order.

You can get the keys, order them however you like, then iterate the dictionary values that way.

keys = myDict.keys()
keys = sorted(keys)  # order them in some way
for k in keys:
   v = myDict[k]

Frustule

I think you wanted keys.sort(). sorted() returns a new list but does not modify the passed object. – Linkous

Answer 3

You can't; dicts are unsortable. Use an OrderedDict if you need an ordered dictionary.

Banwell

dict is sortable (you can always apply sorted to it) but the default order of keys cannot be influenced. – Monadism

what if the keys are numeric & inserted incrementally eg:1,2,3,....10. So in this case, the order is preserved right? – Diphase