Table of Contents

## Problem Formulation

π¬ **Programming Challenge**: Given a list of lists (nested list). Find and return the longest inner list from the outer list of lists.

Here are some examples:

`[[1], [2, 3], [4, 5, 6]]`

π`[4, 5, 6]`

`[[1, [2, 3], 4], [5, 6], [7]]`

π`[1, [2, 3], 4]`

`[[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]]`

π`[7, 8, 9, 10]`

Also, you’ll learn how to solve a variant of this challenge.

π¬ **Bonus challenge**: Find only the *length *of the longest list in the list of lists.

Here are some examples:

`[[1], [2, 3], [4, 5, 6]]`

π`3`

`[[1, [2, 3], 4], [5, 6], [7]]`

π`3`

`[[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]]`

π`4`

So without further ado, let’s get started!

## Method 1: max(lst, key=len)

Use Pythonβs built-in `max()`

function with a key argument to find the longest list in a list of lists. Call `max(lst, key=len)`

to return the longest list in `lst`

using the built-in `len()`

function to associate the weight of each list, so that the longest inner list will be the maximum.

Here’s an example:

def get_longest_list(lst): return max(lst, key=len) print(get_longest_list([[1], [2, 3], [4, 5, 6]])) # [4, 5, 6] print(get_longest_list([[1, [2, 3], 4], [5, 6], [7]])) # [1, [2, 3], 4] print(get_longest_list([[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]])) # [7, 8, 9, 10]

A beautiful one-liner solution, isn’t it? π Let’s have a look at a slight variant to check the *length *of the longest list instead.

## Method 2: len(max(lst, key=len))

To get the length of the longest list in a nested list, use the `len(max(lst, key=len))`

function. First, you determine the longest inner list using the `max()`

function with the key argument set to the `len()`

function. Second, you pass this longest list into the `len()`

function itself to determine the maximum.

Here’s an analogous example:

def get_length_of_longest_list(lst): return len(max(lst, key=len)) print(get_length_of_longest_list([[1], [2, 3], [4, 5, 6]])) # 3 print(get_length_of_longest_list([[1, [2, 3], 4], [5, 6], [7]])) # 3 print(get_length_of_longest_list([[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]])) # 4

## Method 3: max(len(x) for x in lst)

A Pythonic way to check the length of the longest list is to combine a generator expression or list comprehension with the `max()`

function without key. For instance, `max(len(x) for x in lst)`

first turns all inner list into length integer numbers and passes this iterable into the `max()`

function to get the result.

Here’s this approach on the same examples as before:

def get_length_of_longest_list(lst): return max(len(x) for x in lst) print(get_length_of_longest_list([[1], [2, 3], [4, 5, 6]])) # 3 print(get_length_of_longest_list([[1, [2, 3], 4], [5, 6], [7]])) # 3 print(get_length_of_longest_list([[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]])) # 4

A good training effect can be obtained by studying the following tutorial on the topic—feel free to do so!

π **Training**: Understanding List Comprehension in Python

## Method 4: Naive For Loop

A not so Pythonic but still fine approach is to iterate over all lists in a `for`

loop, check their length using the `len()`

function, and compare it against the currently longest list stored in a separate variable. After the termination of the loop, the variable contains the longest list.

Here’s a simple example:

def get_longest_list(lst): longest = lst[0] if lst else None for x in lst: if len(x) > len(longest): longest = x return longest print(get_longest_list([[1], [2, 3], [4, 5, 6]])) # [4, 5, 6] print(get_longest_list([[1, [2, 3], 4], [5, 6], [7]])) # [1, [2, 3], 4] print(get_longest_list([[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]])) # [7, 8, 9, 10] print(get_longest_list([])) # None

So many lines of code! π At least does the approach also work when passing in an empty list due to the ternary operator used in the first line.

`lst[0] if lst else None`

If you need a refresher on the ternary operator, you should check out our blog tutorial.

π **Training Tutorial**: The Ternary Operator — A Powerful Python Device

β **Note**: If you need the length of the longest list, you could simply replace the last line of the function with `return len(longest)`

, and you’re done!

## Summary

You have learned about four ways to find the longest list and its length from a Python list of lists (nested list):

**Method 1**:`max(lst, key=len)`

**Method 2**:`len(max(lst, key=len))`

**Method 3**:`max(len(x) for x in lst)`

**Method 4**: Naive For Loop

I hope you found the tutorial helpful, if you did, feel free to consider joining our community of likeminded coders—we do have lots of free training material!

π Also, check out our tutorial on finding the general maximum of a list of lists—it’s a slight variation!

π**Recommended Tutorial**: Python Find Longest List in Dict of Lists

While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.

To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.

His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.