How to Use itertools.combinations in Python: Simple Guide

Use itertools.combinations(iterable, r) to get all possible combinations of length r from the iterable. It returns an iterator of tuples, each tuple representing one combination.

Syntax

The itertools.combinations function takes two main arguments:

• iterable: The collection of items you want to combine.
• r: The number of items in each combination.

It returns an iterator that produces tuples of length r, each tuple being a unique combination without repetition.

python

import itertools

combinations = itertools.combinations(iterable, r)

Example

This example shows how to get all 2-item combinations from a list of fruits.

python

import itertools

fruits = ['apple', 'banana', 'cherry']
combos = itertools.combinations(fruits, 2)

for combo in combos:
    print(combo)

Output

('apple', 'banana')
('apple', 'cherry')
('banana', 'cherry')

Common Pitfalls

Common mistakes include:

• Using combinations when you want permutations (order matters).
• Setting r larger than the length of the iterable, which returns no results.
• Forgetting that combinations returns an iterator, so you can only loop through it once unless converted to a list.

python

import itertools

items = [1, 2, 3]

# Wrong: r larger than length
print(list(itertools.combinations(items, 4)))  # Outputs []

# Right: r within length
print(list(itertools.combinations(items, 3)))  # Outputs [(1, 2, 3)]

Output

[]
[(1, 2, 3)]

Quick Reference

Function	Description
itertools.combinations(iterable, r)	All unique combinations of length r from iterable
itertools.permutations(iterable, r)	All ordered arrangements of length r from iterable
itertools.combinations_with_replacement(iterable, r)	Combinations allowing repeated elements

Key Takeaways

Use itertools.combinations to get unique groups of items without caring about order.

The second argument r must be less than or equal to the length of the iterable.

combinations returns an iterator, so convert to list if you need to reuse results.

For order-sensitive arrangements, use permutations instead of combinations.

combinations_with_replacement allows repeated elements in combinations.