Sort Dict by Values in Python 3.6+

Question

I was looking for a method to sort a dictionary in Python with its values, after a few attempts, is what it comes:

a = {<populated dict...>}
a = {v: k for k, v in a.items()}
a = {v: k for k, v in sorted(a.items())}

This code seems to work, but I think it's poor for performance, is there a better way?

Moppet

Possible duplicate of How can I sort a dictionary by key? – Tragedy

@newbie, not sure how a question that sorts by key is a dupe for one asking for sort by value? – Jepson

Answer 1

By default, the dictionary is sorted based on keys, but the sorted function takes a function as a parameter using which you can alter the behaviour for program.

d={'a':6,'b':4,'k':3}
print(sorted(d)) 

sorted_by_values= sorted(d,key=lambda x:d[x])
print(sorted_by_values)

Roach

Answer 2

You do not need to do the double key/value swap, you can do this:

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

(sorted DOCS)

Test Code:

data = dict(a=1, b=3, c=2)
print(data)
data_sorted = {k: v for k, v in sorted(data.items(), key=lambda x: x[1])}
print(data_sorted)

Results:

From CPython 3.6, and guaranteed in all python implementations 3.7+:

{'a': 1, 'b': 3, 'c': 2}
{'a': 1, 'c': 2, 'b': 3}

Jepson

In what way will this sort the dictionary? – Historiography

@PeterWood, In CPython 3.6 and Python 3.7+ Dicts are ordered. – Jepson

I don't like it as an approach. Insertion sort is a feature since 3.6 but The order-preserving aspect of this new implementation is considered an implementation detail and should not be relied upon from the documentation – Tragedy

@newbie, your are technically correct. However, for 99% of the world, CPython is Python. And for this feature at least, in 3.7 it is no longer an implementation detail. – Jepson

Answer 3

The following code works for me. Not sure how efficient is this.

sorted_list_by_value=sorted(data_dict, key=data_dict.__getitem__)

Scrambler

Answer 4

Similar to the top answer but without the dict comprehension, resulting in marginally faster and shorter code:

Code:

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

sorted returns a list of tuples, which dict() then uses as its key/value pairs.

Example:

a = {'a': 1, 'b': 3, 'c': 2}
a = dict(sorted(a.items(), key=lambda x: x[1]))
print(a)

Results:

{'a': 1, 'c': 2, 'b': 3}

Works in CPython 3.6+ and all Pythons 3.7+.

Wrapper

Answer 5

otherwise create a list of keys in the order you want.

from collections import OrderedDict
d = {1: 1, 2:4, 5:2, 9:3}
sorted_dict = OrderedDict(sorted(d.items(),key=lambda x: x[1]))

Edited to include sort into sorted dict

Blackleg

OrderedDict preserves the order in which the keys are inserted and does not store them in increasing or decreasing order by value. – Keyser

In old version of python dicts were explicitly unordered so if you wanted to sort a dict you'd need to do something like OrderedDict(sorted(d,key=lambda x:d[x])) the accepted answer isn't a "sorted dict" it's a sorted iterable of tuples – Blackleg