What are itertools in Python?

Itertools is a module in python, it is used to iterate over data structures that can be stepped over using a for-loop. Such data structures are also known as iterables.

This module incorporates functions that utilize computational resources efficiently. Using this module also tends to enhance the readability and maintainability of the code.

The itertools module needs to be imported prior to using it in the code.


Types of iterators:

  • Infinite iterators: run indefinitely (i.e they will continue to run forever if you do not put a stopping condition).
  • Finite iterators: terminate (i.e you do not run the risk of getting into an infinite loop).
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Examples


1. Infinite Iterators

These include count, cycle and repeat.

Count: This function takes two optional arguments and returns an iterator.

  • The first argument determines the starting value.
  • The second argument dictates how much the value will increment in each step.

If the arguments are not provided then (by default) start will be set to 0 and step will be set to 1.

#print the first four even numbers
import itertools
result = itertools.count(start = 0, step = 2)
for number in result:
# termination condition
    if number < 8:
        print (number)
    else:
        break

Cycle: This function takes in an iterable and goes over it indefinitely​.

# print 2 three times
import itertools
result = itertools.cycle('12345')
counter = 0
for number in result:
# termination condition
    if counter < 10 :
        print (number)
        counter = counter + 1
    else:
        break

Repeat: This function takes an iterable and iterates over it indefinitely. There are two main differences between cycle and repeat:

  • Cycle: Goes through the iterable, one element at a time, but repeat takes the whole iterable each time.
  • Repeat: Takes an optional times parameter that can be used as a termination condition.
# print hello two times
import itertools
result = itertools.repeat('hello', times = 2)
for word in result:
    print (word)

2. Finite Iterators

These include but are not limited to chain, compress and dropwhile.

Chain: This function accepts a variable number of iterables and loops through all of them, one by one.

# iterate over three lists
import itertools
list_one = ['a', 'b', 'c']
list_two =['d', 'e', 'f']
list_three = ['1', '2', '3']
result = itertools.chain(list_one, list_two, list_three)
for element in result:
  print (element)

Compress: This function takes in an iterable and a selector, and returns an iterable with only those items for which the corresponding selector value is true.

#find the names of people who have the flu
import itertools
names = ['Alice', 'James', 'Matt']
have_flu = [True, True, False]
result = itertools.compress(names, have_flu)
for element in result:
  print (element)

DropWhile: An iterable and a function (predefined or lambda) is passed to it. Based on the condition inside the function, dropwhile keeps on dropping values from the iterable until it encounters the first element that evaluates to false.

import itertools
my_list = [0, 0, 1, 2, 0]
result = itertools.dropwhile(lambda x: x == 0, my_list)
for elements in result:
  print (elements)

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