Iterators & itertools

Introduction

Any object with __iter__ + __next__ is an iterator. itertools provides production-ready building blocks: chain, islice, groupby, product.

Understanding the topic

Core concepts to understand:

• Iterator: __iter__ + __next__.
• Generators are iterators.
• itertools is the Swiss-army knife.
• StopIteration ends iteration.

Syntax reference

Visual flow / code:

import itertools as it

# Infinite + take first N
list(it.islice(it.count(1), 5))     # [1,2,3,4,5]

# Chain iterables
list(it.chain([1,2], [3,4]))         # [1,2,3,4]

# Group consecutive by key
data = [("a",1), ("a",2), ("b",3)]
for k, group in it.groupby(data, key=lambda x: x[0]):
    print(k, list(group))

# Cartesian product
list(it.product([1,2], ["a","b"]))   # [(1,'a'),(1,'b'),(2,'a'),(2,'b')]

Execution workflow

Real-world use

itertools is one of the most underused gems of the stdlib — replaces dozens of lines of manual looping with clear, fast primitives.

Best practices

• Reach for itertools before writing nested loops.
• Use islice instead of slicing big iterables.

Hands-on exercise

Interview preparation — practice these questions:

• Iterator vs iterable?
• Show 3 itertools functions.
• What's StopIteration?

Summary

In summary: Iter protocol = simple contract. itertools = composable building blocks.