Can I trust the order of a dict to remain the same each time it is iterated over?

Question

I have the following three strings (they exist independently but are displayed here together for convenience):

from mx2.x.org (mx2.x.org. [198.186.238.144])
            by mx.google.com with ESMTPS id g34si6312040qgg.122.2015.04.22.14.49.15
            (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128);
            Wed, 22 Apr 2015 14:49:16 -0700 (PDT)

from HQPAMAIL08.x.org (10.64.17.33) by HQPAMAIL13.x.x.org
 (10.34.25.11) with Microsoft SMTP Server (TLS) id 14.2.347.0; Wed, 22 Apr
 2015 17:49:13 -0400

from HQPAMAIL13.x.org ([fe80::7844:1f34:e8b2:e526]) by
 HQPAMAIL08.iadb.org ([fe80::20b5:b1cb:9c01:aa86%18]) with mapi id
 14.02.0387.000; Wed, 22 Apr 2015 17:49:12 -0400

I'm looking to populate a dict with some values based on the reversed (bottom to top) order of the strings. Specifically, for each string, I'm extracting the IP address as an index of sorts, and then the full string as the value.

Given that order is important, I decided to go with lists, and initially did something like this (pseudocode, with the above bunch of text):

IPs =[]
fullStrings =[]
for string in strings:
    IPs.append[$theIpAddressFoundInTheString]
    fullstrings.append[$theWholeString]

resulting in the following two lists (again, just an illustration):

IPs ['198.186.238.144', '10.64.17.33', 'fe80::7844:1f34:e8b2:e526']

fullstrings ['from mx2.x.org (mx2.x.org. [198.186.238.144])
                by mx.google.com with ESMTPS id g34si6312040qgg.122.2015.04.22.14.49.15
                (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128);
                Wed, 22 Apr 2015 14:49:16 -0700 (PDT)', 'from HQPAMAIL08.x.org (10.64.17.33) by HQPAMAIL13.x.x.org
     (10.34.25.11) with Microsoft SMTP Server (TLS) id 14.2.347.0; Wed, 22 Apr
     2015 17:49:13 -0400', 'from HQPAMAIL13.x.org ([fe80::7844:1f34:e8b2:e526]) by
     HQPAMAIL08.x.org ([fe80::20b5:b1cb:9c01:aa86%18]) with mapi id
     14.02.0387.000; Wed, 22 Apr 2015 17:49:12 -0400']

This has worked fine up until a point, but now as I begin populating a dict with values in these lists (at hardcoded indices), comparing against values in other lists (again at hardcoded indices) etc., not only does debugging become a pain, the code becomes unsustainable.

I'm beginning to rewrite using a dict (returning a dict where the IP addresses are the keys and the full strings are the values). Then I will perform operations like:

for k,v in myDictOfIpsAndStrings:
    anotherDict[$someHardcodedText] = k
    anotherDict[$otherHardcodedText] = v        

Here's my concern: can I be sure that the dict, any time it is iterated over, will always be done in the order in which the dict was created? If not, is my only option to revert back to lists (and the tedious and brittle length comparisons, assignments inherent in doing so) etc.?

I know that a dict is, by its very nature, unsorted. And I know of the sorted function, but I'm not looking to sort they keys by any descending/ascending order etc. rather it's about maintaining (somehow) the order in which the dict was created.

Answer 1

can I be sure that the dict, any time it is iterated over, will always be done in the order in which the dict was created?

No, a dict is unordered, and will lay out its ordering however the particular implementation decides to.

>>> d = {3: 'c', 2: 'b', 1: 'a'}
>>> d
{1: 'a', 2: 'b', 3: 'c'}

See, immediately after I created the dict the order changed.

If you want to ensure you have a deterministic, controllable order, you can use a collections.OrderedDict

>>> from collections import OrderedDict
>>> d = OrderedDict([(3, 'c'), (2, 'b'), (1, 'a')])
>>> d
OrderedDict([(3, 'c'), (2, 'b'), (1, 'a')])

You can still access the OrderedDict in the conventions you are used to

>>> d[3]
'c'
>>> d.get(3)
'c'

Note that you do not have to insert all of the elements upon creation. You can insert them one at a time if you want.

>>> d = OrderedDict()
>>> d[3] = 'c'
>>> d[2] = 'b'
>>> d[1] = 'a'
>>> d[4] = 'd'
>>> d
OrderedDict([(3, 'c'), (2, 'b'), (1, 'a'), (4, 'd')])

Answer 2

You should not rely on the iteration order of a dict. The only way you can get any stable and repeatable ordering is by doing something like:

for key in sorted(yourdict):
   more code here

That will give you a stable ordering, but probably not the one you want.

You probbaly want to use an OrderedDict