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Itertools

The module itertools is a standard library that contains several functions that are useful in functional programming. Advanced iteration functions capabilities for:

Infinite iterators

Iterator Arguments Syntax Example
count() start, [step] start, start+step, start+2*step, … count(10) --> 10 11 12 13 14 ...
cycle() p p0, p1, … plast, p0, p1, … cycle('ABCD') --> A B C D A B C D ...
repeat() elem [,n] elem, elem, elem, … endlessly or up to n times repeat(10, 3) --> 10 10 10
  • The function count() counts up infinitely from a value.

from itertools import count
for i in count(3):
  print(i)
  if i>=11:
    break

3
4
5
6
7
8
9
10
11


import itertools
# use count to create a simple counter
count1 = itertools.count(100, 10)
print(next(count1))
print(next(count1))
print(next(count1))

100
110
120

  • The function cycle() infinitely iterates through an iterable (for instance a list or string).

import itertools
# cycle iterator can be used to cycle over a collection
seq1 = ["Joe", "John", "Mike"]
cycle1 = itertools.cycle(seq1)
print(next(cycle1))
print(next(cycle1))
print(next(cycle1))
print(next(cycle1))

Joe
John
Mike
Joe

  • The function repeat() repeats an object, either infinitely or a specific number of times.

A common use for repeat is to supply a stream of constant values to map or zip

list(map(pow, range(10), itertools.repeat(2)))

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Iterators terminating on the shortest input sequence

All list

Iterator Arguments Syntax Example
accumulate() p [,func] p0, p0+p1, p0+p1+p2, … accumulate([1,2,3,4,5]) --> 1 3 6 10 15
dropwhile() pred, seq seq[n], seq[n+1], starting when pred fails dropwhile(lambda x: x<5, [1,4,6,4,1]) --> 6 4 1
takewhile() pred, seq seq[0], seq[1], until pred fails takewhile(lambda x: x<5, [1,4,6,4,1]) --> 1 4
chain() p, q, … p0, p1, … plast, q0, q1, … chain('ABC', 'DEF') --> A B C D E F
chain.from_iterable() iterable p0, p1, … plast, q0, q1, … chain.from_iterable(['ABC', 'DEF']) --> A B C D E F
groupby() iterable[, key] sub-iterators grouped by value of key(v)
zip_longest() p, q, … (p[0], q[0]), (p[1], q[1]), … zip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-
compress() data, selectors (d[0] if s[0]), (d[1] if s[1]), … compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F
filterfalse() pred, seq elements of seq where pred(elem) is false filterfalse(lambda x: x%2, range(10)) --> 0 2 4 6 8
islice() seq, [start,] stop [, step] elements from seq[start:stop:step] islice('ABCDEFG', 2, None) --> C D E F G
pairwise() iterable (p[0], p[1]), (p[1], p[2]) pairwise('ABCDEFG') --> AB BC CD DE EF FG
starmap() func, seq func(seq[0]), func(seq[1]), … starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000
tee() it, n it1, it2, … itn splits one iterator into n
  • accumulate() - returns a running total of values in an iterable

from itertools import accumulate
nums= list(accumulate(range(8)))
print(nums)

[0, 1, 3, 6, 10, 15, 21, 28]


# accumulate creates an iterator that accumulates values
vals = [10,20,30,40,50,40,30]
acc = itertools.accumulate(vals, max)
print(list(acc))

[10, 20, 30, 40, 50, 50, 50]

  • takewhile() - takes items from an iterable while a predicate function remains true

from itertools import accumulate, takewhile
nums= list(accumulate(range(8)))
print(nums)
print(list(takewhile(lambda x:x<=6,nums)))

[0, 1, 3, 6, 10, 15, 21, 28]
[0, 1, 3, 6]

  • dropwhile() - drops elements from the iterable as long as the predicate is true

# advanced iteration functions in the itertools package
import itertools

def testFunction(x):
    return x < 40


def main():

    # dropwhile and takewhile will return values until
    # a certain condition is met that stops them
    print(list(itertools.dropwhile(testFunction, vals)))
    print(list(itertools.takewhile(testFunction, vals)))


if __name__ == "__main__":
    main()

[40, 50, 40, 30]
[10, 20, 30]

  • chain() - combines several iterables into one long one

# advanced iteration functions in the itertools package

import itertools
def main():
    # use chain to connect sequences together
    x = itertools.chain("ABCD", "1234")
    print(list(x))

if __name__ == "__main__":
    main()

['A', 'B', 'C', 'D', '1', '2', '3', '4']

Combinatoric iterators

  • Product : cartesian product, equivalent to a nested for-loop

print(list(itertools.product('ABC', repeat=2)))

[('A', 'A'), ('A', 'B'), ('A', 'C'), ('B', 'A'), ('B', 'B'), ('B', 'C'), ('C', 'A'), ('C', 'B'), ('C', 'C')]

  • Permutations: r-length tuples, all possible orderings, no repeated elements

print(list(itertools.permutations('ABC', 2)))

[('A', 'B'), ('A', 'C'), ('B', 'A'), ('B', 'C'), ('C', 'A'), ('C', 'B')]


from itertools import product, permutations
letters=("A","B")
print(list(product(letters,range(2))))
print(list(permutations(letters)))

[('A', 0), ('A', 1), ('B', 0), ('B', 1)]
[('A', 'B'), ('B', 'A')]

  • Combinations : r-length tuples, in sorted order, no repeated elements

print(list(itertools.combinations('ABC', 2)))

[('A', 'B'), ('A', 'C'), ('B', 'C')]


print(list(itertools.combinations_with_replacement('ABC', 2)))

[('A', 'A'), ('A', 'B'), ('A', 'C'), ('B', 'B'), ('B', 'C'), ('C', 'C')]