Dictionary in python with order I set at start

Question

I'm making dictionary:

d = {"server":"mpilgrim", "database":"master"}
d['mynewkey'] = 'mynewvalue'

But when I display it I saw that this dict is reversed.

print(d)

{'mynewkey': 'mynewvalue', 'database': 'master', 'server': 'mpilgrim'}

How to reverse it back?

Or if it is true that dictionary is not sortable what I must to use to have collection where the order of that informations matters?

score 20 · Answer 1 · answered Dec 20 '10 at 15:17

20

from collections import OrderedDict

d = OrderedDict()
d["server"] = "mpilgrim"
d["database"] = "master"
d['mynewkey'] = 'mynewvalue'

print(d)

OrderedDict([('server', 'mpilgrim'), ('database', 'master'), ('mynewkey', 'mynewvalue')])

answered Dec 20 '10 at 15:17

Aphex

7,390
5
33
54

No, it's not ok, because, at start "server":"mpilgrim" was first. Now is second – user278618 Dec 20 '10 at 15:19
Right, I fixed the key insertion once I noticed. For `OrderedDicts`, you need to add keys using the [ ] operator (`__setitem__`). Insertion order is preserved. – Aphex Dec 20 '10 at 15:21

score 18 · Accepted Answer · answered Dec 20 '10 at 15:10

18

Dictionary are unordered (the order is deterministic, but depends on a handful of factors you don't even think of and shouldn't care about - hash of the keys, order of insertion, collisions, etc). In Python 2.7+, use collections.OrderedDict. If you must use an older version, there are various implementations google can point you to.

answered Dec 20 '10 at 15:10

The easiest implementation being a list of tuples, especiall if speed isn't an issue. – Katriel Dec 20 '10 at 16:04
Yes, that's easiest, but it performs *horribly*. Especially when it's used with algorithms that assume "real" (i.e. `O(1)` read/write/`in`) dicts – Dec 20 '10 at 16:20

score 4 · Answer 3 · answered Jan 11 '17 at 17:29

4

If you are using python 3.6 dictionaries are now ordered by insertion order

https://www.python.org/dev/peps/pep-0468/

Otherwise (as other people have said) I would recommend collections.OrderedDict

answered Jan 11 '17 at 17:29

andy boot

11,355
3
53
66

Be careful, this is an implementation detail of CPython in 3.6 and may not hold in other implementations. In 3.7 it is guaranteed by the language spec. – Soren Bjornstad Jul 27 '19 at 01:10
As @SorenBjornstad says: In Python 3.8.x dicts are not ordered by insertion order. – petermeissner Oct 22 '20 at 20:44

score 1 · Answer 4 · edited May 23 '17 at 12:02

Your dictionnary is not quite reverted. It is sorted partly according to the hash of you keys. You can't change this order (not without using a custom dictionary). In python 2.7 or later you can use an ordered dictionary (collections.OrderedDict).

In earlier version you can use the following recipe.

Also have a look at this question : What is the best ordered dict implementation in python?

Dictionary in python with order I set at start

4 Answers4