Question

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Why is python ordering my dictionary like so?

I have this fully working code, but it acts strangely: items in dictionary are iterated not sequentially but somehow randomly, why is that?:

#!/usr/bin/python

newDict = {}
lunar = {'a':'0','b':'0','c':'0','d':'0','e':'0','f':'0'}
moon = {'a':'1','d':'1','c':'1'}

for il, jl in lunar.items():
    print "lunar: " + il + "." + jl 
    for im, jm in moon.items():
        if il == im:
            newDict[il] = jm
            break
        else:
            newDict[il] = jl

print newDict

Output:

lunar: a.0
lunar: c.0
lunar: b.0
lunar: e.0  
lunar: d.0
lunar: f.0
{'a': '1', 'c': '1', 'b': '0', 'e': '0', 'd': '1', 'f': '0'}

Answer 1

Python dict is not ordered. For performance reasons, it is more efficient for the implementation to forget the order in which you added items.

As the documentation states:

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.

If you need an ordered dictionary, you can use OrderedDict.