Flattens nested iterables into a single iterator for sequential iteration.

To express the operation of flattening nested iterables in Python code.

When you have a list of lists (or any iterable of iterables) and need to iterate over all elements as a flat sequence.

Data processing, loops, functional programming, when flattening nested structures before iteration.

itertools.chain.from_iterable(<iterable_of_iterables>)

itertools.chain.from_iterable

nested_list

• list list of lists nested list iterable iterator

• itertools.chain(*nested_list) sum(nested_list
• []) list comprehension [x for sub in nested_list for x in sub]

Using itertools.chain(*nested_list) when sub‑iterables are not all iterable, causing TypeError Forgetting that the result is an iterator, not a list; forgetting to consume it or convert to list if needed

itertools.chain (for chaining separate iterables) itertools.chain.from_iterable (for flattening one level of nesting) itertools.chain.from_iterable vs itertools.chain.from_iterable on already‑flat iterables (no effect)

Confusing itertools.chain (which chains separate iterables) with itertools.chain.from_iterable (which flattens one level of nesting).

• itertools.chain
• itertools.chain.from_iterable
• itertools.groupby
• itertools.product

Returns a lazy iterator; does not materialize a list unless explicitly converted.

Signals the intent to flatten a nested structure for efficient, memory‑friendly iteration.

Think of chaining the inner iterables together: ‘chain from iterable’.

Commonly used in data‑processing pipelines where nested lists need to be flattened before further transformation.

itertools.chain.from_iterable → itertools.chain for chaining separate iterables, or itertools.groupby for grouping after flattening.