Itertools

• A collection of iterator tools that works in a fast, concise and memory efficient way.
• It's sequential data you can loop over.
• Holds only one object in memory at a time.
• A standard library! Nothing to install to use this! Just import itertools!

Count

• Just returns an iterator that counts

This will result to an infinite loop. Don't run this! You should always have something to specify how many times this should count.

import itertools

counter = itertools.count()

for num in counter:
    print(num)

You can run this because it's obvious that print() is there four times.

import itertools

counter = itertools.count()

print(next(counter))
print(next(counter))
print(next(counter))
print(next(counter))
# 0
# 1
# 2
# 3

For the next example, let's say we wanted this data to be paired up with an index value. This is a nice use case for this itertools.count() function.

data = [100, 200, 300, 400]

daily_data = list(zip(itertools.count(), data))
# zip(): combines two iterables and pairs the values together.
# zip() itself needs to be iterated, for example with a loop
# or by turning it into a list.
# this is also lazy iteration.

print(daily_data)
# [(0, 100), (1, 200), (2, 300), (3, 400)]

Now we can also pass some arguments into this count() function, to start at a different place or step by a different amount. The counter also supports going backwards or in decimals.

counter = itertools.count(start=5, step=2.5)

print(next(counter))
print(next(counter))
print(next(counter))
print(next(counter))
# 5
# 7.5
# 10.0
# 12.5

zip_longest

Like zip(), it pairs iterables together, but doesn't stop until the longest iterable is exhausted. It will fill the missing values with placeholders. By default (without the fillvalue parameter) it is None

data = [100, 200, 300, 400]

daily_data = list(itertools.zip_longest(range(10), data, fillvalue="pwat"))

print(daily_data)
#[(0, 100), (1, 200), (2, 300), (3, 400), (4, 'pwat'), (5, 'pwat'), (6, 'pwat'), (7, 'pwat'), (8, 'pwat'), (9, 'pwat')]

cycle

Also returns an iterator that goes on forever. It takes an iterable as an argument, and iterate through it over and over.

counter = itertools.cycle([1, 2, 3])

print(next(counter))
print(next(counter))
print(next(counter))
print(next(counter))
print(next(counter))
print(next(counter))
# 1
# 2
# 3
# 1
# 2
# 3

Pretty simple... But there's a lot you could do with this! For creating a switch for example, you could create a cycle with two values.

repeat

Yet another infinite iterator. Also very simple: just takes an input and repeats it indefinitely. With the times argument, we can set a limit on how many times it will repeat it.

counter = itertools.repeat(2, times=3)

print(next(counter))
print(next(counter))
print(next(counter))
print(next(counter))

After the third print statement, there will be a stop iteration error. But a for loop will avoid that error:

print([i for i in itertools.repeat(2, 3)])
# [2, 2, 2]

Supplies a stream of constant values to be used with map or zip:

print(list(map(pow, range(10), itertools.repeat(2))))
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

A very fast way to loop over a fixed number of times:

for _ in itertools.repeat(None, 3):
    print("pwat")

This is faster than:

for i in range(3):
    print("pwat")

Another example:

counter = itertools.repeat(7, times=7)

print(list(counter))
# [7, 7, 7, 7, 7, 7, 7]