[Tut] Zip With List Output Instead of Tuple | Most Pythonic Way

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Zip With List Output Instead of Tuple | Most Pythonic Way

Short answer: Per default, the

Code:

zip()

function returns a zip object of tuples. To obtain a list of lists as an output, use the list comprehension statement

Code:

[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

Code:

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

Code:

l1, l2, ...

. How ot zipthe i-th elements of those lists together and obtain a listof lists?

Example: Given two lists

Code:

[1, 2, 3, 4]

and

Code:

['Alice', 'Bob', 'Ann', 'Liz']

and you want the list of lists

Code:

[[1, 'Alice'], [2, 'Bob'], [3, 'Ann'], [4, 'Liz']]

.

Code:

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:

https://trinket.io/embed/python/07d4e000fe

Exercise: Create a new list

Code:

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

Code:

list()

constructor.

Code:

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).

Code:

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 zipobject to a list and appendthis list to the nested list

Code:

zipped

.

Code:

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

Code:

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.

Code:

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

Code:

list()

constructor to the

Code:

map()

function to convert each tuple in the zip object to a list. You can then convert the map object to a list.

Code:

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.

Discussion

The most Pythonic way to create a list of lists by zipping together multiple lists is the list comprehension statement

Code:

[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:

Code:

[list(x) for x in zip(l1, l2, l3, ..., ln)]

. The

Code:

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.

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