itertools

Compositional iterators for fast looping. Memory-efficient because each function returns an iterator that lazily produces values.

Source-of-record: Modules/itertoolsmodule.c, itertools docs.

Infinite iterators

Iterator	Output
count(start=0, step=1)	start, start+step, start+2*step, ...
cycle(iterable)	Forever cycle through iterable.
repeat(object, times=None)	object forever, or times times.

Terminating iterators

Iterator	Output
accumulate(iterable, func=operator.add, *, initial=None)	Running totals.
batched(iterable, n, *, strict=False)	Tuples of up to n elements (3.12+).
chain(*iterables) / chain.from_iterable(it)	Flatten one level.
compress(data, selectors)	data where selector is truthy.
dropwhile(predicate, iterable)	Skip while predicate is true, then yield rest.
filterfalse(predicate, iterable)	Inverse of filter.
groupby(iterable, key=None)	Adjacent-run grouping.
islice(iterable, stop) / islice(iterable, start, stop, step=1)	Slice an iterator.
pairwise(iterable)	Overlapping pairs (3.10+).
starmap(func, iterable)	func(*item) per item.
takewhile(predicate, iterable)	Yield while predicate is true.
tee(iterable, n=2)	n independent iterators sharing buffer.
zip_longest(*iterables, fillvalue=None)	Zip padded to longest.

Combinatoric iterators

Iterator	Output
product(*iterables, repeat=1)	Cartesian product.
permutations(iterable, r=None)	r-length permutations.
combinations(iterable, r)	r-length combinations.
combinations_with_replacement(iterable, r)	With replacement.

Recipes

itertools ships docstring recipes (not module members): grouper, roundrobin, partition, iter_except. Copy into your code; not part of the importable surface.

Gopy status

Area	State
All infinite iterators	Complete.
All terminating iterators incl. batched, pairwise	Complete.
All combinatoric iterators	Complete.
C-fast paths	Complete via module/_itertools.

Reference

• CPython 3.14: itertools.
• Modules/itertoolsmodule.c.
• module/_itertools/. gopy port.