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How does the key argument in python's sorted function work?

Say I have

votes = {'Charlie': 20, 'Able': 10, 'Baker': 20, 'Dog': 15}

I understand

print(sorted(votes.items(), key=lambda x: x[1]))

will lead to

[('Able', 10), ('Dog', 15), ('Baker', 20), ('Charlie', 20)]

But how does this work?

like image 371
Cheese Avatar asked Aug 26 '15 23:08

Cheese


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How does sorted key work?

One of the most powerful components of sorted() is the keyword argument called key . This argument expects a function to be passed to it, and that function will be used on each value in the list being sorted to determine the resulting order. The resulting order is a list with a string order of shortest to longest.

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2 Answers

The function you pass in to key is given each of the items that are being sorted, and returns a "key" that Python can sort by. So, if you want to sort a list of strings by the reverse of the string, you could do this:

list_of_strings.sort(key=lambda s: s[::-1])

This lets you specify the value each item is sorted by, without having to change the item. That way, you don't have to build a list of reversed strings, sort that, then reverse them back.

# DON'T do this

data = ['abc', 'def', 'ghi', 'jkl']
reversed_data = [s[::-1] for s in data]
reversed_data.sort()
data = [s[::-1] for s in reversed_data]

# Do this

data.sort(key=lambda s: s[::-1])

In your case, the code is sorting each item by the second item in the tuple, whereas normally it would initially sort by the first item in the tuple, then break ties with the second item.

like image 193
Cyphase Avatar answered Sep 18 '22 22:09

Cyphase


>>> votes = {'Charlie': 20, 'Able': 10, 'Baker': 20, 'Dog': 15}

If we apply .items() on the votes dictionary above we get:

>>> votes_items=votes.items()
>>> votes_items
[('Charlie', 20), ('Baker', 20), ('Able', 10), ('Dog', 15)]
#a list of tuples, each tuple having two items indexed 0 and 1

For each tuple, the first index [0] are the strings ('Charlie','Able','Baker','Dog') and the second index [1] the integers (20,10,20,15).

print(sorted(votes.items(), key = lambda x: x[1])) instructs python to sort the items(tuples) in votes using the second index [1] of each tuple, the integers, as the basis of the sorting.

Python compares each integer from each tuple and returns a list that has ranked each tuple in ascending order (this can be reversed with the reverse=True argument) using each tuple's integer as the key to determine the tuple's rank,

Where there is a tie in the key, the items are ranked in the order they are originally in the dictionary. (so ('Charlie', 20) is before ('Baker', 20) because there is a 20==20 tie on the key but ('Charlie', 20) comes before ('Baker', 20) in the original votes dictionary).

The output then is:

 [('Able', 10), ('Dog', 15), ('Charlie', 20), ('Baker', 20)]

I hope this makes it easier to understand.

like image 43
Aishwarya Shukla Avatar answered Sep 17 '22 22:09

Aishwarya Shukla