+++ slug = "itertools" title = "Itertools" section = "Iteration" summary = "itertools composes lazy iterator streams." doc_path = "/library/itertools.html" see_also = [ "iterators", "generator-expressions", "sentinel-iteration", "comprehension-patterns", ] +++

The itertools module contains tools for composing iterator streams: combining, slicing, grouping, and repeating values without changing the consumer protocol.

Many itertools functions are lazy. They describe work to do later instead of building a list immediately, so helpers such as islice() are useful when taking a finite window.

Iterator pipelines let each step stay small: one object produces values, another transforms them, and a final consumer such as list() or a loop pulls values through the pipeline.

:::program

import itertools

counter = itertools.count(10)
print(list(itertools.islice(counter, 3)))

pages = itertools.chain(["intro", "setup"], ["deploy"])
print(list(pages))

scores = [7, 10, 8, 10]
high_scores = itertools.compress(scores, [score >= 9 for score in scores])
print(list(high_scores))

:::

:::cell count() can produce values forever, so islice() takes a finite window. Nothing is materialized until list() consumes the iterator.

import itertools

counter = itertools.count(10)
print(list(itertools.islice(counter, 3)))

[10, 11, 12]

:::

:::cell chain() presents several iterables as one stream. This avoids building an intermediate list just to loop over combined inputs.

pages = itertools.chain(["intro", "setup"], ["deploy"])
print(list(pages))

['intro', 'setup', 'deploy']

:::

:::cell Iterator helpers compose with ordinary Python expressions. compress() keeps items whose corresponding selector is true.

scores = [7, 10, 8, 10]
high_scores = itertools.compress(scores, [score >= 9 for score in scores])
print(list(high_scores))

[10, 10]

:::

:::note

• itertools composes producer and transformer streams.
• Iterator pipelines avoid building intermediate lists.
• Use islice() to take a finite piece from an infinite iterator.
• Convert to a list only when you need concrete results. :::