Sorting nested python dictionary

Question

I have a python dictionary with the following format.

{'KEY': (NUMBER, [DATE1, DATE2]}

For example, {'a': (10, ['2010/04/11','2009/03/11'])}

How can I sort this dictionary by the DATE1(the first element in the list)?

EDIT

Can I sorted again when the date is the same? For example.

{'a': (10, ['2010/04/11','2009/03/11'])}
{'b': (20, ['2010/04/11','2009/03/10'])}
{'c': (100, ['2009/01/01'])}

-->

{'b': (20, ['2010/04/11','2009/03/10'])} // this comes first as 20 > 10
{'a': (10, ['2010/04/11','2009/03/11'])}
{'c': (100, ['2009/01/01'])}

What do you mean by saying sort this dictionary? Do you have a lot of dictionaries in this format? — Pol
– Pol, Commented Mar 15, 2011 at 4:20

Ignacio Vazquez-Abrams · Accepted Answer · 2011-03-15 04:19:30Z

4

Dictionaries are unsortable. If you want a sorted list of 2-tuples:

sorted(D.iteritems(), key=lambda x: x[1][1][0])

Pass the expression to datetime.strptime() if you want it in date order instead of string order (not that there is a huge difference given the existing date format...).

answered Mar 15, 2011 at 4:19

Ignacio Vazquez-Abrams

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3 Comments

David Z Over a year ago

It's worth mentioning that the reason dictionaries are unsortable is that they don't maintain any notion of an ordering of their elements.

prosseek Over a year ago

I added one more question. Can I sort in the sorted list when the date is the same? Thanks.

Ignacio Vazquez-Abrams Over a year ago

Sure. Just return a tuple as the key. key=lambda x: (x[1][1][0], -x[1][0])

joshayers · Accepted Answer · 2011-03-15 06:18:28Z

3

If you're using Python 2.7 or later, you can use the new collections.OrderedDict class. Along with Ignacio's code for doing the actual sorting, that should give you what you're looking for.

Say that a is your current dictionary, then

OrderedDict(sorted(a.items(), key=lambda x: (x[1][1][0], -x[1][0])))

will return a sorted version.

answered Mar 15, 2011 at 6:18

joshayers

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