Python dictionary: are keys() and values() always the same order?

Question

It looks like the lists returned by keys() and values() methods of a dictionary are always a 1-to-1 mapping (assuming the dictionary is not altered between calling the 2 methods).

For example:

>>> d = {'one':1, 'two': 2, 'three': 3}
>>> k, v = d.keys(), d.values()
>>> for i in range(len(k)):
    print d[k[i]] == v[i]

True
True
True

If you do not alter the dictionary between calling keys() and calling values(), is it wrong to assume the above for-loop will always print True? I could not find any documentation confirming this.

In CPython 3.7 (and up, presumably) you may rely on the iteration order of a dictionary matching insertion order. mail.python.org/pipermail/python-dev/2017-December/151283.html — BallpointBen
– BallpointBen, Commented Apr 19, 2018 at 18:47
@BallpointBen it's in CPython 3.6 and up and all other Python implementations starting with Python 3.7 — user3064538
– user3064538, Commented May 19, 2020 at 14:43

David Fraser · Accepted Answer · 2016-12-19 13:03:36Z

480

Found this:

If items(), keys(), values(), iteritems(), iterkeys(), and itervalues() are called with no intervening modifications to the dictionary, the lists will directly correspond.

On 2.x documentation and 3.x documentation.

edited Dec 19, 2016 at 13:03

David Fraser

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answered May 7, 2009 at 15:45

nosklo

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5 Comments

Shaohua Li Over a year ago

apparently the statement in the 3.x documentation is clearer: "the order of items will directly correspond"

EliadL Over a year ago

This text is missing in the 3.7 documentation. I can only assume it's because "Dict keeps insertion order" in 3.7: mail.python.org/pipermail/python-dev/2017-December/151283.html

chepner Over a year ago

Correct; before, the iteration order wasn't predictable, but it was consistent. Predictable iteration order implies consistency.

robertspierre Over a year ago

@chepner "consistent" meaning that if I call values() after keys(), it is guaranteed that the order of values() is the same as the order of keys()?

Fritz Over a year ago

@robertspierre: Yes, anything else I would consider inconsistent.

vvvvv · Accepted Answer · 2020-12-10 14:11:43Z

107

Yes, what you observed is indeed a guaranteed property -- keys(), values() and items() return lists in congruent order if the dict is not altered. iterkeys() &c also iterate in the same order as the corresponding lists.

edited Dec 10, 2020 at 14:11

vvvvv

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answered May 7, 2009 at 14:49

Alex Martelli

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1 Comment

Ville Over a year ago

However, in case the list is altered, Pandas dataframe offers an alternative where items can be updated or deleted, and the order and the index locations of the items in a dict-like structure remain fixed.

Bjorn · Accepted Answer · 2014-06-03 14:50:20Z

51

Yes it is guaranteed in python 2.x:

If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond.

edited Jun 3, 2014 at 14:50

answered May 7, 2009 at 15:20

Bjorn

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Comments

score 22 · Accepted Answer · 2019-04-24 17:58:56Z

22

Yes. Starting with CPython 3.6, dictionaries return items in the order you inserted them.

Ignore the part that says this is an implementation detail. This behaviour is guaranteed in CPython 3.6 and is required for all other Python implementations starting with Python 3.7.

edited Apr 24, 2019 at 17:58

answered Dec 16, 2017 at 19:16

user3064538

Comments

Sassa NF · Accepted Answer · 2018-06-25 09:06:40Z

7

Good references to the docs. Here's how you can guarantee the order regardless of the documentation / implementation:

k, v = zip(*d.iteritems())

answered Jun 25, 2018 at 9:06

Sassa NF

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1 Comment

Good Fit Over a year ago

In python3.x, use dict.items()

sykloid · Accepted Answer · 2009-05-07 15:10:48Z

5

According to http://docs.python.org/dev/py3k/library/stdtypes.html#dictionary-view-objects , the keys(), values() and items() methods of a dict will return corresponding iterators whose orders correspond. However, I am unable to find a reference to the official documentation for python 2.x for the same thing.

So as far as I can tell, the answer is yes, but only in python 3.0+

answered May 7, 2009 at 15:10

sykloid

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Comments

Koen Bok · Accepted Answer · 2009-05-07 15:21:05Z

4

For what it's worth, some heavy used production code I have written is based on this assumption and I never had a problem with it. I know that doesn't make it true though :-)

If you don't want to take the risk I would use iteritems() if you can.

for key, value in myDictionary.iteritems():
    print key, value

answered May 7, 2009 at 15:21

Koen Bok

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Comments

Gregory Sky · Accepted Answer · 2022-06-30 15:55:08Z

I would agree with others that in python 3.6+ it should stay the same if unaffected by user.

an example from my code few days ago:

ips = { '001' : '199.250.178.14', '002' : '199.18.2.89', '003' : '109.251.63.21' }
def run(self):
    for x, y in self.ips.items():
                try:
                    subprocess.check_call(
                    ['ping', '-n', '1', y],
                    stdout=DEVNULL,  # suppress output
                    stderr=DEVNULL
                    )
                except subprocess.CalledProcessError:
                    nextServer = ('HUB ' + x + ' is OFFLINE   ' + " IP:   " + y)

and the output is always of the same order, exactly as i inputted it i.e. only offline servers will show up in order: HUB 003 is OFFLINE IP: 109.251.63.21 becasue first two are reachable

Hope that clears it out

jlansey · Accepted Answer · 2017-07-12 16:10:16Z

-1

I wasn't satisfied with these answers since I wanted to ensure the exported values had the same ordering even when using different dicts.

Here you specify the key order upfront, the returned values will always have the same order even if the dict changes, or you use a different dict.

keys = dict1.keys()
ordered_keys1 = [dict1[cur_key] for cur_key in keys]
ordered_keys2 = [dict2[cur_key] for cur_key in keys]

answered Jul 12, 2017 at 16:10

jlansey

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