How to Iterate over Multiple Lists in Parallel in Python

Sometimes you may need to iterate over multiple lists together or simultaneously. There are different ways to do this in python. In this article, we will look at how to iterate multiple lists in parallel in Python.

How to Iterate over Multiple Lists in Parallel in Python

You can use either zip() or itertools.zip_longest() functions to iterate over multiple lists in parallel in Python.

1. Using zip()

zip() function traverses both lists in parallel but stops the moment any of the individual list is exhausted. In other words, zip() stops when the shortest list in the group stops. Here is an example to iterate over 2 lists in parallel using zip() function. It returns a list of tuples, where each tuple contains 1 item from each of the lists being iterated.

>>> num1 = [1, 2, 3]
>>> num2 = [4, 5]
>>> for (a, b) in zip(num1, num2):
     print (a, b)

     
(1, 4)
(2, 5)

Please note, zip() function runs only till the smallest list runs.

Also is you are using really long lists, then you should use itertools.izip() instead to save memory and for better performance. This is because in python 2.x izip() returns an iterator while zip() returns list.

>>> import itertools
>>> num1 = [1, 2, 3]
>>> num2 = [4, 5]
>>> for (a, b) in itertools.izip(num1, num2):
     print (a, b)

     
(1, 4)
(2, 5)

2. Using itertools.izip_longest()

If you want python to traverse till the longest list is exhausted, then use itertools.izip_longest() function instead.

>>> import itertools
>>> num1 = [1, 2, 3]
>>> num2 = [4, 5]
>>> for (a, b) in itertools.izip_longest(num1, num2):
     print (a, b)

     
(1, 4)
(2, 5)
(3, None)

In this article, we have learnt different ways to traverse two lists in parallel. zip() function iterates the lists till any of them gets exhausted. itertools.izip_longest() runs till all lists are exhausted. If you are using python 2.x and your lists are huge then you need to use itertools.izip() that returns an iterator, thereby saving memory.

Please note, in python 3.x, izip() and izip_longest() are not there as both zip() and zip_longest() return iterators. In python 2.x, zip() used to return lists.