Implementing for loop on a dictionary in Python

Question

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python looping seems to not follow sequence?
In what order does python display dictionary keys?

d = {'x': 9, 'y': 10, 'z': 20}
for key in d: 
    print d[key]

The above code give different outputs every time I run it. Not exactly different outputs, but output in different sequence. I executed the code multiple times using Aptana 3.

First Execution Gave: 10 9 20

Second Execution Gave: 20 10 9

I also executed the code in an online IDE http://labs.codecademy.com. There the output was always 10 9 20

I just wanted to know why is this. Ideally it should have printed 9 10 20 every time I execute the above code. Please Explain.

Python dictionaries do not have a fixed order. – Martijn Pieters Jan 31 '13 at 12:49 — Martijn Pieters, Jan 31 '13 at 12:49

score 3 · Accepted Answer · answered Jan 31 '13 at 12:52

3

A dictionary is a mapping of keys to values; it does not have an order.

You want a collections.OrderedDict:

collections.OrderedDict([('x', 9), ('y', 10), ('z', 20)])
Out[175]: OrderedDict([('x', 9), ('y', 10), ('z', 20)])

for key in Out[175]:
    print Out[175][key]

Note, however, that dictionary ordering is deterministic -- if you iterate over the same dictionary twice, you will get the same results.

answered Jan 31 '13 at 12:52

Katriel

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@root I have one ipython open for scratch =) – Katriel Jan 31 '13 at 16:38

score 1 · Answer 2 · answered Jan 31 '13 at 12:50

A dictionary is a collection that is not ordered. So in theory the order of the elements may change on each operation you perform on it. If you want the keys to be printed in order, you will have to sort them before printing(i.e. collect the keys and then sort them).

Implementing for loop on a dictionary in Python

2 Answers2