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Keeping Dictionaries in Order

Problem

You want to create a dictionary, and you also want to control the order of items when iterating or serializing.

Solution

To control the order of items in a dictionary, you can use an OrderedDict from the collections module.



In [1]:

    
from collections import OrderedDict

d = OrderedDict()
d['foo'] = 1
d['bar'] = 2
d['spam'] = 3
d['grok'] = 4

# Outputs "foo 1", "bar 2", "spam 3", "grok 4"
for key in d: 
    print(key, d[key])









    



foo 1
bar 2
spam 3
grok 4

An OrderedDict can be particularly useful when you want to build a mapping that you may want to later serialize or encode into a different format. For example, if you want to precisely control the order of fields appearing in a JSON encoding, first building the data in an OrderedDict will do the trick:



In [2]:

    
import json
json.dumps(d)









    Out[2]:





'{"foo": 1, "bar": 2, "spam": 3, "grok": 4}'

An OrderedDict internally maintains a doubly linked list that orders the keys according to insertion order.
Be aware that the size of an OrderedDict is more than twice as large as a normal dic‐ tionary due to the extra linked list that’s created.