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.

Answer 1

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.