Python loop dict in assigned order

Question

I've run into a problem with looping through a dictionary. The following code:

d = {}
d['a'] = 1
d['b'] = 2
d['c'] = 3
for k,v in d.iteritems():
    print k,v

Results in:

a 1
c 3
b 2

But the desired result is:

a 1
b 2
c 3

Does anyone know how to fix the loop somehow to preserve the order that the elements were assigned in? Thank you!

score 2 · Accepted Answer · answered Mar 01 '14 at 23:21

In Python, normal dictionaries were designed to be unordered. Meaning, they should not be used here.

Instead, you should use a collections.OrderedDict:

>>> from collections import OrderedDict
>>> d = OrderedDict()
>>> d['a'] = 1
>>> d['b'] = 2
>>> d['c'] = 3
>>> for k,v in d.iteritems():
...     print k,v
...
a 1
b 2
c 3
>>>

Unlike a normal dictionary, an OrderedDict is guaranteed to preserve the order in which the elements were assigned.

Sorry for the delay, it made me wait 10 minutes.. – Charles Mar 01 '14 at 23:43 — Charles, Mar 01 '14 at 23:43

score 1 · Answer 2 · answered Mar 01 '14 at 23:22

A dictionary does not preserve order, so you need to sort the keys:

for k,v in sorted(d.iteritems,key=lambda x: x[0]):
    print k,v

Alternatively, use a collections.OrderedDict object which internally maintains a list of keys in insertion order:

import collections
d = collections.OrderedDict()
...
for k,v in d.iteritems():
    print k,v

Python loop dict in assigned order

2 Answers2