Basic flatten
Input
[[1, 2], [3, 4]]
Output
[1, 2, 3, 4]
Each inner list is expanded one level into the resulting list.
Full lesson preview
Flatten a single-level list of lists
Flatten only one level of nesting: turn a list of lists (and/or tuples and scalars) into a single list of elements.
Python practice16 minLists & TuplesIntermediateLast updated March 17, 2026
Problem statement
Write a function flatten_once(nested) that accepts a list which may contain other lists or tuples as elements and returns a new list with exactly one level of flattening applied.
For each element in the input list:
- If the element is a list or tuple, extend the output with its elements (one level deep).
- Otherwise, append the element as-is.
Do not perform deep flattening: if an inner element contains nested lists beyond the first level, leave those nested structures intact as elements.
Task
Implement a function that flattens one level of nesting for a list containing sublists/tuples and other elements.
Examples
Input format
A single Python list which may contain lists, tuples, or other objects as elements.
Output format
A list with one level of flattening applied.
Constraints
Only flatten one level. Preserve the order of elements. Result must be a list.
Samples
Sample 1
Input
[['a'], ['b', 'c'], []]
Output
['a', 'b', 'c']
Empty inner lists contribute nothing; other inner lists are expanded.