Question

How do I sort a dictionary by value in Python?

I have a dictionary of values read from two fields in a database: a string field and a numeric field. The string field is unique, so that is the key of the dictionary.

I can sort on the keys, but how can I sort based on the values?

Answer 1

Problem Statement – Here are the major tasks that are needed to be performed.

Create a dictionary and display its keys alphabetically.

Display both the keys and values sorted in alphabetical order by the key.

Same as part (ii), but sorted in alphabetical order by the value.

Approach –

Load the Dictionary and perform the following operations:

First, sort the keys alphabetically using key_value.iterkeys() function.

Second, sort the keys alphabetically using sorted (key_value) function & print the value corresponding to it.

Third, sort the values alphabetically using key_value.iteritems(), key = lambda (k, v) : (v, k))

Python 3.7+ or CPython 3.6

Dicts preserve insertion order in Python 3.7+. Same in CPython 3.6, but it's an implementation detail.

>>> x = {1: 2, 3: 4, 4: 3, 2: 1, 0: 0}

>>> {k: v for k, v in sorted(x.items(), key=lambda item: item[1])}

{0: 0, 2: 1, 1: 2, 4: 3, 3: 4}

or

>>> dict(sorted(x.items(), key=lambda item: item[1]))

{0: 0, 2: 1, 1: 2, 4: 3, 3: 4}

Older Python

It is not possible to sort a dictionary, only to get a representation of a dictionary that is sorted. Dictionaries are inherently orderless, but other types, such as lists and tuples, are not. So you need an ordered data type to represent sorted values, which will be a list—probably a list of tuples.

For instance,

import operator

x = {1: 2, 3: 4, 4: 3, 2: 1, 0: 0}

sorted_x = sorted(x.items(), key=operator.itemgetter(1))

sorted_x will be a list of tuples sorted by the second element in each tuple. dict(sorted_x) == x.

And for those wishing to sort on keys instead of values:

import operator

x = {1: 2, 3: 4, 4: 3, 2: 1, 0: 0}

sorted_x = sorted(x.items(), key=operator.itemgetter(0))

In Python3 since unpacking is not allowed we can use

x = {1: 2, 3: 4, 4: 3, 2: 1, 0: 0}

sorted_x = sorted(x.items(), key=lambda kv: kv[1])

If you want the output as a dict, you can use collections.OrderedDict:

import collections

sorted_dict = collections.OrderedDict(sorted_x)