The itertools module in Python is a set of tools that are designed to be fast and use minimal memory, which is especially helpful when working with large amounts of data.

One of the most useful tools in this module is itertools.chain(), which allows you to combine multiple iterables (e.g. lists, tuples, etc.) in a highly efficient way, without the need to create new lists.

In this article, we'll take a look at how itertools.chain() works and why it's a great option for making your Python code more efficient and easier to read.

What is itertools.chain()?

itertools.chain() is a function that lets you combine multiple iterables (e.g. lists, sets, or dictionaries) into a single iterable.

This is especially helpful when you need to go through multiple sequences one after the other, without having to merge them into a single large container.

How Does itertools.chain() Work?

The itertools.chain() function takes multiple iterables (e.g. lists, tuples, etc.) as input and returns a single iterator.

This iterator goes through each of the input iterables one at a time, producing (or "yielding") each element until it has gone through all of the elements in all of the input iterables.

Here's a simple example to illustrate how it works:

from itertools import chain

# Define three lists
list1 = [1, 2, 3]
list2 = [4, 5, 6]
list3 = [7, 8, 9]

# Using chain to iterate over all lists sequentially
for number in chain(list1, list2, list3):
    print(number)

Output:

1
2
3
4
5
6
7
8
9

In this next example, we'll see how itertools.chain() can be used to easily go through a list, a tuple, and a set, without the need for multiple loops or complex code to combine them:

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Python