Storing Python dictionary entries in the order they are pushed

Question

A Python dictionary is stored in no particular order (mappings have no order), e.g.

>>> myDict = {'first':'uno','second':'dos','third':'tres'}
myDict = {'first':'uno','second':'dos','third':'tres'}
>>> myDict
myDict
{'second': 'dos', 'third': 'tres', 'first': 'uno'}

While it is possible to retrieve a sorted list or tuple from a dictionary, I wonder if it is possible to make a dictionary store the items in the order they are passed to it, in the previous example this would mean having the internal ordering as {'first':'uno','second':'dos','third':'tres'} and no different.

I need this because I am using the dictionary to store the values as I read them from a configuration file; once read and processed (the values are altered), they have to be written to a new configuration file in the same order as they were read (this order is not alphabetical nor numerical).

Any thoughts?

Please notice that I am not looking for secondary ways to retrieve the order (like lists), but of ways to make a dictionary be ordered in itself (as it will be in upcoming versions of Python).

http://stackoverflow.com/questions/1867861/python-dictionary-keep-keys-values-in-same-order-as-declared — Ofri Raviv, Dec 09 '09 at 08:16
Since you're trying to maintain order, it's not really a dictionary in the first place. You're doing too many things. You might want both dictionary (for the mapping) and list (to retain the order). Nothing wrong with that. — S.Lott, Dec 09 '09 at 11:46
@S.Lott: You are right - no need to force a data structure into a different behavior than it was designed for. I will create my own or use the ones already described here. Thanks. — Escualo, Dec 09 '09 at 17:15

YOU · Accepted Answer · 2009-12-09T08:14:48.433

34

Try python 2.7 and above, probably 3.1, there is OrderedDict

http://www.python.org/

http://python.org/download/releases/2.7/

>>> from collections import OrderedDict
>>> d = OrderedDict([('first', 1), ('second', 2),
...                  ('third', 3)])
>>> d.items()
[('first', 1), ('second', 2), ('third', 3)]

PEP 372: Adding an ordered dictionary to collections

edited Dec 09 '09 at 08:14

answered Dec 09 '09 at 08:09

YOU

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Nice. But keep in mind that is a new feature of an upcoming Python release and is not available in older versions. – Andreas Kraft Dec 09 '09 at 08:17
... but I'm stuck with 2.5.1 :( – Escualo Dec 09 '09 at 08:20
3

Or get the source code of the OrderedDict from this patch: http://bugs.python.org/issue5397 – Georg Schölly Dec 09 '09 at 08:20
2

If you look at the PEP (http://www.python.org/dev/peps/pep-0372/) you'll find a list of implementations of ordered dictionaries for older pythons. If performance isn't an issue I'd suggest using their own sample implementation of odict (http://dev.pocoo.org/hg/sandbox/raw-file/tip/odict.py), which you'll be able to swap for the standard library one if you ever upgrade. – Michael Dunn Dec 09 '09 at 08:28
It's important as well, if its possible to push to an ordered Dict, this is in some applications essential. Otherweise you have to build-up the OrderedDict everytime you have a new element from a dict. – user1767754 Apr 08 '15 at 05:56
great, I was looking for this only. Thanks – Kavyajeet Bora Oct 26 '20 at 16:50

score 4 · Answer 2 · answered Dec 09 '09 at 08:07

4

Use a list to hold the key order

answered Dec 09 '09 at 08:07

Ofri Raviv

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quick and dirty. alas, not very compact (can get brittle). – Daren Thomas Dec 09 '09 at 08:15
This works, but I am looking for a more "natural" way, if there is one. It seems that the OrderedDict suggested by S.Mark is the alternative I was looking for. Unfortunately I am stuck with Python 2.5.1 :( – Escualo Dec 09 '09 at 08:19
1

Not if you encapsulate the dict and list in a single object with dict interface. Which, by the way, is what many Ordered Dict implementations do (don't know about the implementation actually adopted in 2.7) – Vinko Vrsalovic Dec 09 '09 at 08:20

score 3 · Answer 3 · answered Dec 09 '09 at 11:10

3

Implementations of order-preserving dictionaries certainly do exist.

There is this one in Django, confusingly called SortedDict, that will work in Python >= 2.3 iirc.

answered Dec 09 '09 at 11:10

Mauve

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Just noticed this is going to be deprecated in Django 1.9. – imns Apr 22 '14 at 13:33

score 2 · Answer 4 · answered Dec 09 '09 at 08:08

2

Dictionaries in Python are implemented as hash tables, which is why the order appears random. You could implement your own variation of a dict that sorts, but you'd lose out on the convenient syntax. Instead, keep track of the order of the keys, too.

Initialization:

keys = []
myDict = {}

While reading:

myDict[key] = value
keys.append(key)

While writing:

for key in keys:
  print key, myDict[key]

answered Dec 09 '09 at 08:08

dmazzoni

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My keys are not in alphabetical or numerical order :( They can be anything. – Escualo Dec 09 '09 at 08:11
You wouldn't need to lose out on any syntax. Simply implement special methods such as __get__() and __set__() etc so enable use of syntax like dict['key'] and dict['key1'] = newvalue. – Isaac Dec 09 '09 at 08:21

score 2 · Answer 5 · answered Jan 30 '15 at 07:32

Rather Than Explaining The Theoretical Part I'll Give A Simple Example.

>>> from collections import OrderedDict
>>> my_dictionary=OrderedDict()
>>> my_dictionary['foo']=3
>>> my_dictionar['aol']=1
>>> my_dictionary
OrderedDict([('foo', 3), ('aol', 1)])

score -2 · Answer 6 · answered Nov 14 '13 at 11:37

-2

There is a very short answer to that.. do this--

dictCopy=yourdictname.copy()

then use the dictCopy , it will be in the same order.

answered Nov 14 '13 at 11:37

user2984414

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Dictionaries in Python have no ordering, thus the question. – Escualo Nov 14 '13 at 18:35

Storing Python dictionary entries in the order they are pushed

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