all Python 3Iterators & generatorsterm

itertools.chain

itertools

Walks through several iterables one after another as if they were a single one.

Syntax

from itertools import chain
chain(a, b, c)

Unlike a + b, chain copies nothing: items are pulled one at a time as you iterate, memory stays constant, and it accepts any iterables — sets, files, generators — not just lists of one type. The gap between chain(*matrix) and chain.from_iterable(matrix) matters: the star expands the outer sequence into arguments immediately, while from_iterable consumes it lazily and therefore survives very long or endless outer sources. What you get back is an ordinary single-pass iterator, so there is no second walk over it.

Examples

from itertools import chain
result = list(chain([1,2],[3,4],[5,6]))
print(result)  # → [1,2,3,4,5,6]

from itertools import chain
# chain.from_iterable
matrix = [[1,2],[3,4],[5,6]]
flat = list(chain.from_iterable(matrix))
print(flat)  # → [1,2,3,4,5,6]

from itertools import chain
# Объединение строк
words = list(chain('hello', ' ', 'world'))
print(''.join(words))  # → hello world

from itertools import chain
# С генераторами
def gen1(): yield 1; yield 2
def gen2(): yield 3; yield 4
print(list(chain(gen1(), gen2())))  # → [1,2,3,4]

from itertools import chain
# chain vs list + list
a = range(1000000); b = range(1000000)

from itertools import chain
# chain(a,b) — без создания списка в памяти!

See also

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