Mixed elements
Input
[1, [2, 3], 4]
Output
[1, 2, 3, 4]
The inner list [2, 3] is flattened into the outer list. Other elements stay the same.
Full lesson preview
Flatten One-Level List
Flatten only one level of nested lists, leaving deeper nested lists intact.
Python practice17 minLoops & IterationIntermediateLast updated March 15, 2026
Problem statement
Write a function that takes a list which may contain elements that are themselves lists. The function should return a new list where any element that is a list is replaced by its elements (one-level flattening). Do not recursively flatten deeper nested lists — they should remain as elements. Preserve the order of elements. This practice focuses on loop control, type checking, and constructing new lists.
Task
Create a function that flattens elements that are lists by one level, practicing iteration and type checks.
Examples
Input format
A single list passed to flatten_one_level(lst). Elements may be any type; lists should be flattened one level.
Output format
A new list with one-level flattening applied.
Constraints
Input list length up to 10,000; total number of elements after flattening will also fit in memory. Only flatten Python list instances (type list). Do not flatten tuples or other iterables.
Samples
Sample 1
Input
[[1,2],[3],[4,5]]
Output
[1, 2, 3, 4, 5]
All inner lists are expanded into the returned list.