Question

I have the following code that uses a heap to get the dictionary elements with the highest value, but it’s not returning the expected result:

import heapq
import operator
a={'third': 3, 'fourth': 2, 'first': 5, 'second': 4}
heapq.nlargest(4,a,operator.itemgetter(1))
>>['fourth', 'first', 'third', 'second']

Why is it not returning:

>>['first' , 'second' , 'third' , 'fourth']

?

Thanks.

Answer 1

Take a look at what operator.itemgetter(1) actually does to your dictionary:

>>> map(operator.itemgetter(1), a)
['e', 'o', 'h', 'i']

When you iterate over a dictionary, you iterate over the keys. Since you want the values, use use a.get to fetch the items:

>>> map(a.get, a)
[4, 2, 3, 5]

a.get(key) is a method that works like a[key], so it’s essentially doing this:

>>> [a[key] for key in a]
[4, 2, 3, 5]

And now your code works:

>>> heapq.nlargest(4, a, a.get)
['first', 'second', 'third', 'fourth']