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

The Question :

337 people think this question is useful

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.

The Question Comments :

The Answer 1

384 people think this answer is useful

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.

The Answer 2

90 people think this answer is useful

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.

The Answer 3

49 people think this answer is useful

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.

The Answer 4

8 people think this answer is useful

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.

The Answer 5

7 people think this answer is useful

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

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

The Answer 6

4 people think this answer is useful

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+

The Answer 7

4 people think this answer is useful

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

The Answer 8

-2 people think this answer is useful

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]

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