chain and chain.from_iterable

The chain() function concatenates multiple iterables into a single iterator, while chain.from_iterable() accepts an iterable of iterables. These tools let you process items from several sources as a single stream without copying them into a new collection, preserving memory efficiency and keeping code concise.

chain() - Concatenate Iterables

Pass any number of iterables as separate arguments to chain(). It yields elements from the first iterable until it is exhausted, then proceeds to the next, and so on.

from itertools import chain

list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']
list3 = [10, 20]

result = list(chain(list1, list2, list3))
print(result)

[1, 2, 3, 'a', 'b', 'c', 10, 20]

chain.from_iterable() - Flatten Iterables

Unlike chain(), which requires each iterable as a separate argument, chain.from_iterable() accepts a single iterable of iterables. This is essential when the sub-iterables come from a generator or when their count is not known in advance.

from itertools import chain

lists = [[1, 2], [3, 4], [5, 6]]
result = list(chain.from_iterable(lists))
print(result)

# Flatten a generator expression
nested = ([i, i+10] for i in range(3))
flattened = list(chain.from_iterable(nested))
print(flattened)

[1, 2, 3, 4, 5, 6]
[0, 10, 1, 11, 2, 12]