earn the White PyBites Ninja earn the Yellow PyBites Ninja earn the Orange PyBites Ninja right arrow earn more PyBites Ninja belts and certificates
The best way to learn to code in Python is to actually use the language.

Our platform offers effective Test Driven Learning which will be key to your progress.


Join thousands of Pythonistas and start coding!


Join us on our PyBites Platform
Click here to code!

5 cool things you can do with itertools

Posted by Bob on Thu 12 January 2017 in Modules • 2 min read

What is itertools?

Itertools is a stdlib module that provides functions that create iterators "inspired by constructs from APL, Haskell, and SML". See the docs page, pymotw, as well as this great EuroPython preso.

5 cool things you can do with itertools

I created this notebook so you can follow along. Below a summary:

1. Use product to get all combinations between two iterators

Common game techniques: build a card deck or roll two dices:

import itertools
import random

# from Fluent Python
ranks = [str(n) for n in range(2, 11)] + list('JQKA')
suits = 'S D C H'.split() # spades diamonds clubs hearts
# but instead of double list comprehension, using product
cards = ['{}{}'.format(*p) for p in itertools.product(suits, ranks)]

Another use case: roll 2 dices (SO is your friend):

dice = range(2, 7)
random.choice([p for p in itertools.product(dice, repeat=2)])

# output: 
(5, 2)
(2, 5)
(2, 5)
(6, 5)

2. Show a progress spinner for a console app

From before-mentioned EuroPython preso:

import itertools
import sys
import time

def spinner(seconds):
    """Show an animated spinner while we sleep."""
    symbols = itertools.cycle('-\|/')
    tend = time.time() + seconds
    while time.time() < tend:
        # '\r' is carriage return: return cursor to the start of the line.
        sys.stdout.write('\rPlease wait... ' + next(symbols)) # no newline
        sys.stdout.flush()
        time.sleep(0.1)
    print() # newline

if __name__ == "__main__":
    spinner(3)

3. Use dropwhile to get counts of >= n in a Counter dict

Given a books Counter object, get me books with >= 2 occurences.

I needed this for my tools of titans kata to see which books got recommended more than once by Tim Ferriss' podcast guests.

def get_multiple_mentions(books, keep=2):
    for key, count in itertools.dropwhile(lambda key_count: key_count[1] >= keep, books.most_common()):
        del books[key]
    return books

# filters all books with count (occurence = 1) out:
Counter({'4-hour-workweek-escape-live-anywhere': 2,
        'alchemist-paulo-coelho': 2,
        'atlas-shrugged-ayn-rand': 3,
        'black-swan-improbable-robustness-fragility': 2,
        'checklist-manifesto-how-things-right': 2,
        ...

4. Combinations and permutations

For the difference read this great explanation.

Given a list of friends how many pairs can be formed?

friends = 'bob tim julian fred'.split()
# as these are "Combinatoric generators" I consume them here for example purposes using list()
list(itertools.combinations(friends, 2))

# output:
[('bob', 'tim'),
('bob', 'julian'),
('bob', 'fred'),
('tim', 'julian'),
('tim', 'fred'),
('julian', 'fred')]

How many 4 letter strings can you from 7 letters? (hint: upcoming challenge)

import string
letters = random.sample(string.ascii_uppercase, 7)
len(list(itertools.permutations(letters, 4)))
# = 840 (7 * 6 * 5 * 4)

5. Groupby to count amount of keys for specific value in dict

Count the number of keys for a value, for example count the number of users (keys) that have email as pref (value) in a user_prefs dict.

This example is based on the one I found at pymotw.

# set up dict
users = 'tim bob julian sue fred frank maria'.split()
prefs = 'email phone IM email F2F email phone'.split()
user_prefs = dict(zip(users, prefs))
user_prefs

user_prefs_sorted = sorted(user_prefs.items(), key=itemgetter(1))
for pref, users in itertools.groupby(user_prefs_sorted, key=itemgetter(1)):
    print(pref, list(map(itemgetter(0), users)))

# output: 
F2F ['fred']
IM ['julian']
email ['frank', 'tim', 'sue']
phone ['bob', 'maria']

Other uses?

Where do you use itertools mostly for? Share your experience in the comments below ...


See an error in this post? Please submit a pull request on Github.