How to Use itertools.chain in Python: Syntax and Examples

Use itertools.chain to combine multiple iterables into a single iterable that yields elements from each input one after another. Import it with from itertools import chain and pass your iterables as arguments to chain().

Syntax

The itertools.chain function takes multiple iterable objects as arguments and returns a single iterable that produces elements from the first iterable until it is exhausted, then continues to the next iterable, and so on.

• chain(iter1, iter2, ...): Combines multiple iterables.
• chain.from_iterable(iterable_of_iterables): Takes a single iterable containing iterables and chains them.

python

from itertools import chain

# Basic syntax
combined = chain([1, 2], ['a', 'b'])

# Using from_iterable
combined_from_iterables = chain.from_iterable([[1, 2], ['a', 'b']])

Example

This example shows how to use chain to combine two lists and iterate over all their elements as if they were one sequence.

python

from itertools import chain

list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']

for item in chain(list1, list2):
    print(item)

Output

1
2
3
a
b
c

Common Pitfalls

One common mistake is trying to concatenate iterables using + which only works for sequences like lists, not all iterables. Another is forgetting to import chain from itertools. Also, using chain with non-iterables will cause errors.

Use chain.from_iterable when you have a single iterable containing multiple iterables, instead of passing them as separate arguments.

python

from itertools import chain

# Wrong: trying to add generators (will raise TypeError)
g1 = (x for x in range(3))
g2 = (x for x in range(3,6))

# combined = g1 + g2  # This is invalid

# Right: use chain to combine generators
combined = chain(g1, g2)
print(list(combined))

Output

[0, 1, 2, 3, 4, 5]

Quick Reference

Function	Description
chain(iter1, iter2, ...)	Combine multiple iterables into one sequence
chain.from_iterable(iterable_of_iterables)	Combine iterables contained in a single iterable
Returns	An iterator that yields elements from each iterable in order

Key Takeaways

Use itertools.chain to combine multiple iterables into a single iterable without creating intermediate lists.

Import chain with 'from itertools import chain' before using it.

Use chain.from_iterable when chaining a single iterable containing multiple iterables.

chain works with any iterable, including lists, tuples, generators, and more.

Avoid using '+' to combine non-sequence iterables; chain is the correct tool.