NullException NullException - 22 days ago 5
Python Question

python dictionary sorting in descending order based on values

I want to sort this dictionary d based on value of sub key key3 in descending order. See below:

d = { '123': { 'key1': 3, 'key2': 11, 'key3': 3 },
'124': { 'key1': 6, 'key2': 56, 'key3': 6 },
'125': { 'key1': 7, 'key2': 44, 'key3': 9 },
}


So final dictionary would look like this.

d = { '125': { 'key1': 7, 'key2': 44, 'key3': 9 },
'124': { 'key1': 6, 'key2': 56, 'key3': 6 },
'123': { 'key1': 3, 'key2': 11, 'key3': 3 },
}


My approach was to form another dictionary e from d, whose key would be value of key3 and then use reversed(sorted(e)) but since value of key3 can be same, so dictionary e lost some of the keys and their values. makes sense?

How I can accomplish this? This is not a tested code. I am just trying to understand the logic.

Answer

Dictionaries do not have any inherent order. Or, rather, their inherent order is "arbitrary but not random", so it doesn't do you any good.

In different terms, your d and your e would be exactly equivalent dictionaries.

What you can do here is to use an OrderedDict:

from collections import OrderedDict
d = { '123': { 'key1': 3, 'key2': 11, 'key3': 3 },
      '124': { 'key1': 6, 'key2': 56, 'key3': 6 },
      '125': { 'key1': 7, 'key2': 44, 'key3': 9 },
    }
d_ascending = OrderedDict(sorted(d.items(), key=lambda kv: kv[1]['key3']))
d_descending = OrderedDict(sorted(d.items(), 
                                  key=lambda kv: kv[1]['key3'], reverse=True))

The original d has some arbitrary order. d_ascending has the order you thought you had in your original d but didn't. And d_ascending has the order you want for your e.


If you don't really need to use e as a dictionary, you just want to be able to iterate over the elements of d in a particular order, you can simplify this:

for key, value in sorted(d.items(), key=lambda kv: kv[1]['key3'], reverse=True):
    do_something_with(key, value)

If you want to maintain a dictionary in sorted order across any changes, instead of an OrderedDict, you want some kind of sorted dictionary. There are a number of options available that you can find on PyPI, some implemented on top of trees, others on top of an OrderedDict that re-sorts itself as necessary, etc.