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Table of Contents
Use zip() to traverse two lists simultaneously
What if the list length is inconsistent?
Methods to get indexes during traversal
Home Backend Development Python Tutorial How to iterate over two lists at once Python

How to iterate over two lists at once Python

Jul 09, 2025 am 01:13 AM
python List iteration

A common way to traverse two lists simultaneously in Python is to use the zip() function, which pairs multiple lists in order and takes the shortest as the prevailing; if the length of the list is inconsistent, itertools.zip_longest() can be used to take the longest as the prevailing value; combined with enumerate(), the index can be obtained at the same time. 1. zip() is simple and practical, suitable for paired data iteration; 2. zip_longest() can fill in the default value when dealing with inconsistent lengths; 3. enumerate(zip()) can obtain indexes during traversal, meeting the needs of a variety of complex scenarios.

How to iterate over two lists at once Python

The most common method to iterate through two lists in Python is to use zip() function. It can "compress" multiple lists together and iterate together. This method is simple and practical, especially suitable for processing paired data.

How to iterate over two lists at once Python

Use zip() to traverse two lists simultaneously

zip() is a Python built-in function that can pair two or more lists in order to form an iterable object. You can use it directly in the for loop:

 names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]

for name, age in zip(names, ages):
    print(name, age)

This way you can output in sequence:

How to iterate over two lists at once Python
 Alice 25
Bob 30
Charlie 35

It should be noted that if the lengths of the two lists are inconsistent, zip() will default to the shortest list , and the remaining elements will be ignored.


What if the list length is inconsistent?

If you want to be able to handle all elements when the list length is inconsistent, you can use itertools.zip_longest() :

How to iterate over two lists at once Python
 from itertools import zip_longest

names = ['Alice', 'Bob', 'Charlie']
scores = [85, 90]

for name, score in zip_longest(names, scores):
    print(name, score)

The output result will be:

 Alice 85
Bob 90
Charlie None
  • The missing position is filled with None by default. You can also specify other values ??through fillvalue parameter, such as fillvalue=0 .
  • This method is suitable for situations where all elements need to be processed, such as generating reports, filling data, etc.

Methods to get indexes during traversal

Sometimes you not only need to traverse the values ??of two lists, but also want to get the current index. At this time, you can combine enumerate() and zip() :

 names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]

for i, (name, age) in enumerate(zip(names, ages)):
    print(f"{i}: {name} is {age} years old")

Output:

 0: Alice is 25 years old
1: Bob is 30 years old
2: Charlie is 35 years old

Pay attention to the position of the brackets: enumerate(zip(...)) should pass the result of zip as a whole. In addition, variables must also be written in (name, age) form to be correctly unpacked.


Basically that's it. Use zip() well, plus enumerate() and zip_longest() to meet most needs to traverse multiple lists at the same time.

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