chain

Learn chaining with itertools.chain.

chain

chain is a function from the itertools module that combines several iterables into a single iterable. It gives sequential access without creating a nested list.

Example: list(chain([1, 2], [3, 4])) returns [1, 2, 3, 4]. chain walks all items one by one; it does not produce a new list, it returns an iterator. This is good for memory; merging large data does not create a copy.

Returns an iterator

chain does not return a list directly; it returns an iterator. To see the result you use list() or a for loop. This is memory efficient; all items are not loaded at once.

To flatten nested lists you can use chain.from_iterable([[1, 2], [3, 4]]). This gives the same result as chain(*[[1, 2], [3, 4]]) but works with a single argument. It is ideal for dynamically generated iterables.

Exercise Merge lists with chain

Start with from itertools import chain. Merge two lists with result = list(chain([1, 2], [3, 4])). Then print the result with print(result).

Show example solution

# Merge two lists with chain
from itertools import chain

result = list(chain([1, 2], [3, 4]))
print(result)

Test yourself

Which module does chain come from?
- functools
- collections
- itertools
- typing
Show answer

itertools — chain is in the itertools module.

chain import line

from itertools import ____

result = list(chain([1, 2], [3, 4]))
print(result)

Show answer

from itertools import chain

result = list(chain([1, 2], [3, 4]))
print(result)

Match the chain parts

chain

Combine function
[1, 2], [3, 4]

Two iterables
list(...)

List the result
from_iterable

Dynamic flatten

Next lesson: count and cycle

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