Why do I need chain and chain.from_iterable?

There are many situations where you need to combine or flatten multiple iterables into a single iterable. Instead of manually concatenating lists or looping through sequences, you can use chain to combine iterables and chain.from_iterable to flatten an iterable of iterables. These functions make your code cleaner, more readable, and more memory efficient, as they process the input lazily.

What does chain do?

The chain function takes multiple iterables as input and returns a single iterator that yields elements from the first iterable until it's exhausted, then proceeds to the next iterable, and so on. It’s great for combining multiple sequences of data into one without creating a new list.

Example 1: Using chain

import itertools

# Combine multiple lists into a single iterator
iterable = itertools.chain([1, 2, 3], ['a', 'b', 'c'], [True, False])

print(list(iterable))

Output:

[1, 2, 3, 'a', 'b', 'c', True, False]

In this example, we combined three lists—one with integers, one with strings, and one with booleans—into a single sequence. chain lets us seamlessly iterate through all of them one by one.

What does chain.from_iterable do?

While chain works well for directly passing multiple iterables, chain.from_iterable is used when you have a single iterable that contains other iterables, and you want to flatten them into one. This function is especially useful when you have a list of lists (or any iterable of iterables) and you want to iterate through all the elements without nesting.

Example 2: Using chain.from_iterable

import itertools

# A list of lists (an iterable of iterables)
data = [[1, 2, 3], ['a', 'b', 'c'], [True, False]]

# Flatten the list of lists
flat_data = itertools.chain.from_iterable(data)

print(list(flat_data))

Output:

[1, 2, 3, 'a', 'b', 'c', True, False]

Here, we used chain.from_iterable to flatten a list of lists into a single iterator. This is a cleaner and more memory-efficient way to flatten iterables compared to manually looping through them.

Combining chain and chain.from_iterable

You can also combine chain and chain.from_iterable to process iterables in more complex ways. For example, you might have a list of iterables and additional standalone elements to append to the sequence. chain handles both scenarios smoothly.

Example 3: Combining chain and chain.from_iterable

import itertools

# A list of iterables and standalone elements
data = [[1, 2], [3, 4]]
additional_data = ['a', 'b', 'c']

# Flatten the data and add additional elements
combined_data = itertools.chain.from_iterable(data)
final_data = itertools.chain(combined_data, additional_data)

print(list(final_data))

Output:

[1, 2, 3, 4, 'a', 'b', 'c']

Here, we first flattened a list of lists using chain.from_iterable, and then appended additional standalone elements using chain.

Conclusion

The chain and chain.from_iterable functions from the itertools module are incredibly helpful for combining multiple iterables into one. Whether you’re dealing with separate iterables or a nested iterable, these functions provide an elegant, memory-efficient solution for flattening and chaining sequences together. They’re great tools for simplifying your code when you need to process data from multiple sources.