Python itertools Module

count() and cycle()

count() produces an infinite sequence of numbers starting from a given value, and cycle() repeats a sequence forever. Both are typically used with a limiting function like itertools.islice or a break condition.

from itertools import count, cycle, islice

for n in islice(count(10, 2), 5):
    print(n, end=" ")
print()

colors = cycle(["red", "green", "blue"])
for _ in range(5):
    print(next(colors), end=" ")

chain() — Joining Iterables

chain() lets you loop over several iterables as if they were one continuous sequence, without copying them into a new combined list.

from itertools import chain

list1 = [1, 2, 3]
list2 = [4, 5, 6]

for n in chain(list1, list2):
    print(n, end=" ")

combinations() and permutations()

combinations() gives every possible unordered grouping of a given size, while permutations() gives every possible ordered arrangement. Both are extremely useful for algorithm practice and probability problems.

from itertools import combinations, permutations

items = ["A", "B", "C"]

print(list(combinations(items, 2)))
print(list(permutations(items, 2)))

groupby() — Grouping Consecutive Items

groupby() groups consecutive items that share the same key. It only groups items that are next to each other, so the input is usually sorted first for meaningful grouping.

from itertools import groupby

words = ["apple", "avocado", "banana", "berry", "cherry"]

for letter, group in groupby(words, key=lambda w: w[0]):
    print(letter, list(group))