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

user_59753 0
Viewed 120230

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.

python
9 Answers

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.

user_171600

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.

user_958100

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.

apphacker_664750

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

k, v = zip(*d.iteritems())
user_19356310
sykora_538500

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
user_564950

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

user_112719270
Related