Sort List of Lists Ascending and then Descending in Python
Last Updated :
15 Jan, 2025
Sorting a list of lists in Python can be done in many ways, we will explore the methods to achieve the same in this article.
Using sorted() with a key
sorted() function is a flexible and efficient way to sort lists. It creates a new list while leaving the original unchanged.
Python
a = [[1, 2, 3], [3, 2, 1], [2, 3, 1]]
# Sort in ascending order based on the first element
asc = sorted(a, key=lambda x: x[0])
print(asc)
# Sort in descending order based on the first element
desc = sorted(a, key=lambda x: x[0], reverse=True)
print(desc)
Output[[1, 2, 3], [2, 3, 1], [3, 2, 1]]
[[3, 2, 1], [2, 3, 1], [1, 2, 3]]
Explanation:
- Key parameter is used to specify the sorting criterion (in this case, the first element).
- Reverse parameter is used for descending order sorting.
Let's explore some more methods and see how we can sort list of lists in ascending then descending order in Python.
Using sort() method
sort() method sorts the list in place, making it memory efficient when a new list is unnecessary.
Python
a = [[1, 2, 3], [3, 2, 1], [2, 3, 1]]
# Sort in ascending order based on the first element
a.sort(key=lambda x: x[0])
print(a)
# Sort in descending order based on the first element
a.sort(key=lambda x: x[0], reverse=True)
print(a)
Output[[1, 2, 3], [2, 3, 1], [3, 2, 1]]
[[3, 2, 1], [2, 3, 1], [1, 2, 3]]
Explanation: This method modifies the original list, saving memory as no new list is created.
Using itemgetter() from operator module
itemgetter() function from the operator module is another efficient way to sort lists, especially for readability.
Python
from operator import itemgetter
a = [[1, 2, 3], [3, 2, 1], [2, 3, 1]]
# Sort in ascending order based on the first element
asc = sorted(a, key=itemgetter(0))
print(asc)
# Sort in descending order based on the first element
desc = sorted(a, key=itemgetter(0), reverse=True)
print(desc)
Output[[1, 2, 3], [2, 3, 1], [3, 2, 1]]
[[3, 2, 1], [2, 3, 1], [1, 2, 3]]
Explanation: itemgetter() function simplifies accessing specific indices in the list, making the code more readable.
Using a custom comparator
We can use a custom comparator with the functools.cmp_to_key() function to define complex sorting behavior.
Python
from functools import cmp_to_key
a = [[1, 2, 3], [3, 2, 1], [2, 3, 1]]
# Custom comparator function
def compare(x, y):
return x[0] - y[0]
# Sort in ascending order based on the first element
asc = sorted(a, key=cmp_to_key(compare))
print(asc)
Output[[1, 2, 3], [2, 3, 1], [3, 2, 1]]
Explanation: Custom comparator function allows for more complex sorting criteria but is generally slower than using a key.
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