How can I iterate through two lists in parallel in Python?

In Python, iterating through two or more lists in parallel is a common task. Python provides several methods to achieve this, each with different behaviors for handling lists of unequal lengths.

Using range() with Index-Based Access

The most basic approach uses range() with the len() function to iterate through both lists using indices ?

Example

When both lists have the same length ?

letters = ['a', 'b', 'c', 'd', 'e']
numbers = [97, 98, 99, 100, 101]
length = len(letters)  # Assuming both lists have same length

for i in range(length):
    print(letters[i], numbers[i])

a 97
b 98
c 99
d 100
e 101

Handling Lists of Different Lengths

When lists have different lengths, use the minimum length to avoid IndexError ?

numbers = [1, 2, 3, 4, 5, 6, 7]
letters = ['a', 'b', 'c']
length = min(len(numbers), len(letters))

for i in range(length):
    print(numbers[i], letters[i])

1 a
2 b
3 c

Using zip() Function

The zip() function is the most Pythonic way to iterate through multiple lists. It automatically stops when the shortest list is exhausted ?

numbers = [1, 2, 3, 4, 5, 6, 7]
letters = ['a', 'b', 'c']

for num, letter in zip(numbers, letters):
    print(num, letter)

1 a
2 b
3 c

Using itertools.zip_longest()

When you need to iterate through all elements of the longest list, use zip_longest(). Missing values are filled with None by default ?

Syntax

zip_longest(iterable1, iterable2, fillvalue=None)

Example

import itertools

numbers = [1, 2, 3, 4, 5, 6, 7]
letters = ['a', 'b', 'c']

for num, letter in itertools.zip_longest(numbers, letters):
    print(num, letter)

1 a
2 b
3 c
4 None
5 None
6 None
7 None

Iterating Three Lists

You can iterate through any number of lists simultaneously ?

import itertools

numbers = [1, 2, 3, 4, 5, 6, 7]
letters = ['a', 'b', 'c', 'd', 'e']
symbols = [10, 11, 12, 13]

for num, letter, symbol in itertools.zip_longest(numbers, letters, symbols):
    print(num, letter, symbol)

1 a 10
2 b 11
3 c 12
4 d 13
5 e None
6 None None
7 None None

Comparison

Conclusion

Use zip() for most parallel iteration tasks as it's clean and efficient. Use zip_longest() when you need to process all elements from the longest list. Use range() only when you specifically need access to indices.