itertools module — Python (Part 2)

3 min readDec 1, 2020

Before starting this blog please check Part 1 — Itertools module. This blog cover terminating iterators on the shortest input sequence.

Topics :

accumulate()
chain()
chain.from_iterables()
compress()
dropwhile()
filterfalse()
islice()

Terminating iterators

accumulate()

accumulate function iterator takes two arguments, iterable and the function. Which would be followed at each iteration of value in iterable. If no function passed , by default addition function takes place.

   itertools.accumulate(iterable[, func])

Below example require itertools and operator module.

 import itertools 
 import operator 
 li1 = [11, 24, 25, 17] 
 List1 = list(itertools.accumulate(li1)) # with no argument
 print(List1) 
 List1 = list(itertools.accumulate(li1, operator.mul))
 print(List1) # Multiplication Operator
 Output:
 [11, 35, 60, 77]
 [11, 264, 6600, 112200]

Next program using max function as a function parameter, which check max value.

 import itertools
 data = [5, 12, 16, 4, 25, 9, 1]
 result = itertools.accumulate(data, max) 
 for i in result:
     print(i)Output:
 5
 12
 16
 16
 25
 25
 25

chain()

This function takes a series of iterables and return them as one long iterable.

    itertools.chain(*iterables) import itertools
 colors = [‘red’, ‘orange’, ‘yellow’, ‘green’, ‘blue’]
 shapes = [‘circle’, ‘triangle’, ‘square’, ‘rectangle’]
 result = itertools.chain(colors, shapes)
 for each in result:
     print(each)Output :
red
orange
yellow
green
blue
circle
triangle
square
rectangleimport itertools
list1 = [1, 2, 3, 4] 
list2 = [1, 5, 6, 8] 
list3 = [9, 10] 
list4 = list(itertools.chain(list1, list2, list3))
print(list4)Output : 
[1, 2, 3, 4, 1, 5, 6, 8, 9, 10]import itertools
L1 = list(itertools.chain(‘ABC’, ‘DEF’, ‘GHI’, ‘JKL’)) 
print(L1)

chain.from_iterable()

Similar to chain() but gets chained items from a single iterable.

from itertools import chain 
data =[‘ABC’, ‘DEF’, ‘GHI’, ‘JKL’] 
L1 = list(chain.from_iterable(data)) 
print(L1)from itertools import chain 
data =[‘123’, ‘456’, ‘78910’]
obj= map(int, list(chain.from_iterable(data))) 
L1 = list(obj)
print(L1)

compress()

This function filters one iterable with another according to the bollean list values passed as selector.

itertools.compress(data, selectors)import itertools
shapes = ['circle', 'triangle', 'square', 'pentagon']
selections = [False, True, True, False]
L1 = itertools.compress(shapes, selections)
for i in L1:
     print(i)

dropwhile()

itertools.dropwhile(predicate, iterable)

Make an iterator that drops elements from the iterable as long as the predicate is true; afterwards, returns every element.

data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
result = itertools.dropwhile(lambda x: x<5, data)
for i in result:
    print(i)

filterfalse()

itertools.filterfalse(predicate, iterable)

This function makes an iterator that filters elements from iterable returning only those for which the predicate is False.

data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
result = itertools.filterfalse(lambda x: x<5, data)
for i in result:
 print(i)Output :
5
6
7
8
9
10

If you send None as function argument, then it will return empty list.

import itertools
data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
result = itertools.filterfalse(None, data)
for i in result:
    print(i)

groupby()

itertools.groupby(iterable, key_func)

This function caluculate the keys for every element in iterable. Returns key and item group by.

islice()

itertools.islice(iterable, start, stop[, step])

This function is very much like slices. This function allows you to cut out a piece of an iterable.

colors = ['red', 'orange', 'yellow', 'green', 'blue',]
few_colors = itertools.islice(colors, 2)
for i in few_colors:
    print(i)