Python's heapq module

December 21, 2018 • 2 min read

Often when working with collections of data, you may want to find the smallest or largest item. It's easy enough to write a function that iterates through the items and returns the smallest or largest one, or use the builtin min(), max(), or sorted() functions. Another interesting way may be implementing a heap (priority) queue.

Python provides a pretty convenient module called heapq that does that for you. heapq comes with a cool set of inbuilt functions that you can read more about in the docs.

Finding the smallest items

Find 3 of the smallest items using the nsmallest function:

import heapq

numbers = [9, 45, 21, 4, 63, 3, 109]

print(heapq.nsmallest(3, numbers)) # [3, 4, 9]

Also, converting a list into a heap using the heapify function automatically sets the smallest item as the first:

import heapq

numbers = [9, 45, 21, 4, 63, 3, 109]

heapq.heapify(numbers)
print(numbers)
# Output: [3, 4, 9, 45, 63, 21, 109]

You can then pop from the heap with heappop():

heapq.heappop(numbers) # 3
print(numbers)
# Output: [4, 45, 9, 109, 63, 21]

heapq.heappop(numbers) # 4
print(numbers)
# Output: [9, 45, 21, 109, 63]

Finding the largest items

Find 3 of the largest items using the nlargest function:

import heapq

numbers = [9, 45, 21, 4, 63, 3, 109]

print(heapq.nlargest(3, numbers)) # [109, 63, 45]

A more practical example

import heapq

people = [
    {'fname': 'John', 'lname': 'Doe', 'age': 30},
    {'fname': 'Jane', 'lname': 'Doe', 'age': 25},
    {'fname': 'Janie', 'lname': 'Doe', 'age': 10},
    {'fname': 'Jane', 'lname': 'Roe', 'age': 22},
    {'fname': 'Johnny', 'lname': 'Doe', 'age': 12},
    {'fname': 'John', 'lname': 'Roe', 'age': 45}
]

oldest = heapq.nlargest(2, people, key=lambda s: s['age'])
print(oldest)
# Output:
# [{'fname': 'John', 'lname': 'Roe', 'age': 45},
# {'fname': 'John', 'lname': 'Doe', 'age': 30}]

youngest = heapq.nsmallest(2, people, key=lambda s: s['age'])
print(youngest)
# Output:
# [{'fname': 'Janie', 'lname': 'Doe', 'age': 10},
# {'fname': 'Johnny', 'lname': 'Doe', 'age': 12}]

It should be noted though that the nsmallest(n, iterable) and nlargest(n, iterable) functions perform best for smaller values of n. For larger values, it is more efficient to use the sorted() function. Also, when n==1, it is more efficient to use the builtin min() and max() functions.

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