Short answer: Per default, the
zip() function returns a zip object of tuples. To obtain a list of lists as an output, use the list comprehension statement
[list(x) for x in zip(l1, l2)] that converts each tuple to a list and stores the converted lists in a new nested list object.
Intermediate Python coders know the
zip() function. But if you’re like me, you’ve often cursed the output of the zip function: first of all, it’s a zip object (and not a list), and, second, the individual zipped elements are tuples. But what if you need a list of lists as output? This article will show you the most Pythonic way of doing this.
Problem: Given a number of lists
l1, l2, .... How ot zip the i-th elements of those lists together and obtain a list of lists?
Example: Given two lists
[1, 2, 3, 4] and
['Alice', 'Bob', 'Ann', 'Liz'] and you want the list of lists
[[1, 'Alice'], [2, 'Bob'], [3, 'Ann'], [4, 'Liz']].
l1 = [1, 2, 3, 4] l2 = ['Alice', 'Bob', 'Ann', 'Liz'] # ... calculate result ... # Output: [[1, 'Alice'], [2, 'Bob'], [3, 'Ann'], [4, 'Liz']]
Here’s a quick overview of our solutions:
Exercise: Create a new list
l3 and change the four methods to zip together all three lists (instead of only two).
Method 1: Generator Expression
The first method uses a generator expression and converts the resulting iterable to a list using the
l1 = [1, 2, 3, 4] l2 = ['Alice', 'Bob', 'Ann', 'Liz'] # Method 1 zipped = list(list(x) for x in zip(l1, l2)) print(zipped) # [[1, 'Alice'], [2, 'Bob'], [3, 'Ann'], [4, 'Liz']]
This is efficient but not the most concise way of accomplishing this task.
Method 2: List Comprehension
A better way is to use list comprehension which is like a generator expression but it creates a list directly without the need to convert an iterable to a list (as in Method 1).
l1 = [1, 2, 3, 4] l2 = ['Alice', 'Bob', 'Ann', 'Liz'] # Method 2: zipped = [list(x) for x in zip(l1, l2)] print(zipped) # [[1, 'Alice'], [2, 'Bob'], [3, 'Ann'], [4, 'Liz']]
Method 3: For Loop and Zip
Coders who don’t like list comprehension and generator expressions (or, who don’t understand these beautiful Python features) often use a simple for loop. In the loop body, you convert each tuple in the zip object to a list and append this list to the nested list
l1 = [1, 2, 3, 4] l2 = ['Alice', 'Bob', 'Ann', 'Liz'] # Method 3: zipped =  for t in zip(l1, l2): zipped.append(list(t)) print(zipped) # [[1, 'Alice'], [2, 'Bob'], [3, 'Ann'], [4, 'Liz']]
This method is readable but less concise.
Method 4: For Loop and Indexing
This method is often used by coders who know neither the
zip() method, nor list comprehension, nor generator expressions: loop over all indices and append a new list obtained by grouping the i-th elements to the list.
l1 = [1, 2, 3, 4] l2 = ['Alice', 'Bob', 'Ann', 'Liz'] # Method 4: zipped =  for i in range(len(l1)): zipped.append([l1[i], l2[i]]) print(zipped) # [[1, 'Alice'], [2, 'Bob'], [3, 'Ann'], [4, 'Liz']]
However, this method is least Pythonic, lengthy, and it works only for equally-sized lists.
Exercise: What happens if the first list has more elements than the second list?
Method 5: Zip() + Map() + List()
A functional way of solving this problem is the map() function that applies a function to each element of an iterable and returns a map object. You can pass the
list() constructor to the
map() function to convert each tuple in the zip object to a list. You can then convert the map object to a list.
l1 = [1, 2, 3, 4] l2 = ['Alice', 'Bob', 'Ann', 'Liz'] # Method 5 print(list(map(list,zip(l1, l2)))) # [[1, 'Alice'], [2, 'Bob'], [3, 'Ann'], [4, 'Liz']]
I don’t recommend this method because functional programming may be difficult to understand for many beginner coders. Guido van Rossum, the creator of Python, disliked functional programming as well.
The most Pythonic way to create a list of lists by zipping together multiple lists is the list comprehension statement
[list(x) for x in zip(l1, l2)]. List comprehension is fast, efficient, concise, and readable. You can also extend it to the general case by adding more lists to the zip function:
[list(x) for x in zip(l1, l2, l3, ..., ln)]. The
zip() function is also robust against lists of different lengths. In this case, the elements up to the maximal index of the shortest list are zipped.
Where to Go From Here?
Enough theory. Let’s get some practice!
Coders get paid six figures and more because they can solve problems more effectively using machine intelligence and automation.
To become more successful in coding, solve more real problems for real people. That’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?
You build high-value coding skills by working on practical coding projects!
Do you want to stop learning with toy projects and focus on practical code projects that earn you money and solve real problems for people?
🚀 If your answer is YES!, consider becoming a Python freelance developer! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.
If you just want to learn about the freelancing opportunity, feel free to watch my free webinar “How to Build Your High-Income Skill Python” and learn how I grew my coding business online and how you can, too—from the comfort of your own home.
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 that has taught exponential skills to millions of coders worldwide. He’s the author of the best-selling programming books Python One-Liners (NoStarch 2020), The Art of Clean Code (NoStarch 2022), and The Book of Dash (NoStarch 2022). Chris also coauthored the Coffee Break Python series of self-published books. He’s a 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.