CODEWITHSUNDEEP
pythondictionary
user2384994
user2384994

Reputation: 1847

Does "for key in dict" in python always iterate in a fixed order?

Does the python code

for key in dict:
    ...

, where dict is a dict data type, always iterate in a fixed order with regrard to key? For example, suppose dict={"aaa":1,"bbb",2}, will the above code always first let key="aaa" and then key="bbb" (or in another fixed order)? Is it possible that the order is random? I am using python 3.3 in ubuntu 13 and let's assume this running environment doesn't change. Thank you.

add one thing: during multiple runs, the variable dict remains unchanged, i.e., generate once and read multiple times.

Upvotes: 7

Views: 3809

Answers (2)

UltraInstinct
UltraInstinct

Reputation: 44444

Intrinsically, a dictionary has no order in which it stores it keys. So you can not rely on the order. (I wouldn't assume the order to be unchanged even when the environment is identical).

One of the few reliable ways:

for key in sorted(yourDictionary.keys()):
    # Use as key and yourDictionary[key]

EDIT: Response to your comment: Python does not store keys in a random fashion. All the documentation says is that, you should not rely on this order. It depends on the implementation how the keys are ordered. What I will say here about your question is: If you are relying on this order, you are probably doing something wrong. In general you should/need not rely on this at all. :-)

Upvotes: 12

atupal
atupal

Reputation: 17210

CPython implementation detail: Keys and values are listed in an arbitrary 
order which is non-random, varies across Python implementations, and 
depends on the dictionary’s history of insertions and deletions.

For more info:http://docs.python.org/2/library/stdtypes.html#dict.items

What's more, You could use collections.OrderedDict to make the order fixed.

Upvotes: 5

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