from itertools import count, cycle, repeat

# count(start, step) - infinite counter
for i in count(10, 2):
    if i > 20:
        break
    print(i)  # 10, 12, 14, 16, 18, 20

# cycle(iterable) - repeat indefinitely
colors = cycle(['red', 'green', 'blue'])
[next(colors) for _ in range(5)]
# ['red', 'green', 'blue', 'red', 'green']

# repeat(element, times) - repeat element
list(repeat('hello', 3))  # ['hello', 'hello', 'hello']

from itertools import permutations, combinations, product

# permutations - all orderings
list(permutations('ABC', 2))
# [('A','B'), ('A','C'), ('B','A'), ('B','C'), ('C','A'), ('C','B')]

# combinations - unique selections
list(combinations('ABC', 2))
# [('A','B'), ('A','C'), ('B','C')]

# product - cartesian product
list(product('AB', '12'))
# [('A','1'), ('A','2'), ('B','1'), ('B','2')]

from itertools import chain, islice, groupby, takewhile, dropwhile

# chain - combine iterables
list(chain([1, 2], [3, 4], [5]))  # [1, 2, 3, 4, 5]

# islice - slice an iterator
list(islice(range(100), 5, 10))  # [5, 6, 7, 8, 9]

# groupby - group consecutive elements
data = [('a', 1), ('a', 2), ('b', 3), ('b', 4)]
for key, group in groupby(data, key=lambda x: x[0]):
    print(key, list(group))
# a [('a', 1), ('a', 2)]
# b [('b', 3), ('b', 4)]

# takewhile/dropwhile
list(takewhile(lambda x: x < 5, [1, 3, 5, 2, 1]))  # [1, 3]
list(dropwhile(lambda x: x < 5, [1, 3, 5, 2, 1]))  # [5, 2, 1]

from itertools import accumulate
import operator

# Running sum
list(accumulate([1, 2, 3, 4]))  # [1, 3, 6, 10]

# Running product
list(accumulate([1, 2, 3, 4], operator.mul))  # [1, 2, 6, 24]

# Running max
list(accumulate([3, 1, 4, 1, 5], max))  # [3, 3, 4, 4, 5]

from itertools import zip_longest, starmap, filterfalse

# zip_longest - zip with fill value
list(zip_longest([1, 2], [3, 4, 5], fillvalue=0))
# [(1, 3), (2, 4), (0, 5)]

# starmap - unpack arguments
list(starmap(pow, [(2, 3), (3, 2), (10, 2)]))  # [8, 9, 100]

# filterfalse - inverse of filter
list(filterfalse(lambda x: x % 2, range(10)))  # [0, 2, 4, 6, 8]