Welcome to Day 34 of your Python journey!
Today, we’ll dive into one of Python’s most powerful standard libraries: itertools.

If you’ve ever wanted to:

• generate combinations,
• create infinite sequences,
• or chain multiple iterables,

…then itertools is your best friend.

🧰 What is itertools?

The itertools module provides a collection of fast, memory-efficient tools that work on iterators and help you loop like a pro.

🔢 1. Infinite Iterators

These functions produce data endlessly (until you stop them).

count(start=0, step=1)

from itertools import count

for i in count(10, 2):
    print(i)
    if i > 20:
        break

Output: 10, 12, 14, 16, 18, 20, 22

cycle(iterable)

Loops through an iterable forever.

from itertools import cycle

for item in cycle(['A', 'B', 'C']):
    print(item)
    break  # Use `break` to stop it

repeat(item, times)

Repeats the same item a number of times.

from itertools import repeat

for item in repeat('Hello', 3):
    print(item)

Output:
Hello
Hello
Hello

🔁 2. Combinatoric Iterators

Perfect for generating permutations, combinations, and Cartesian products.

product()

from itertools import product

colors = ['red', 'blue']
sizes = ['S', 'M']

for combo in product(colors, sizes):
    print(combo)

Output:
('red', 'S'), ('red', 'M'), ('blue', 'S'), ('blue', 'M')

permutations(iterable, r=None)

All possible ordered arrangements.

from itertools import permutations

for p in permutations([1, 2, 3], 2):
    print(p)

combinations(iterable, r)

All possible unordered combinations.

from itertools import combinations

for c in combinations([1, 2, 3], 2):
    print(c)

🔗 3. Utility Iterators

Useful tools for filtering, chaining, slicing, and grouping iterables.

chain()

Joins multiple iterables together.

from itertools import chain

for i in chain([1, 2], ['A', 'B']):
    print(i)

islice()

Slices an iterator like a list.

from itertools import islice

for i in islice(range(100), 10, 15):
    print(i)

Output: 10, 11, 12, 13, 14

compress(data, selectors)

Filters data by truthy values in selectors.

from itertools import compress

data = 'ABCDEF'
selectors = [1, 0, 1, 0, 1, 0]

print(list(compress(data, selectors)))  # ['A', 'C', 'E']

📌 Real Use Cases

🧪 Mini Practice

1. Get all 3-letter permutations of "ABC".
2. Generate all size-color combinations from two lists.
3. Chain 3 lists into one iterator.

🧠 Pro Tip

itertools returns iterators, not lists.
Use list() to consume them, but remember—they are memory efficient only when you don’t convert them.