Lesson guide
What this Python exercise practices
Flatten One-Level List is a intermediate practice lesson that focuses on loops, iteration, counters. It is designed to be solved in about 17 minutes with examples, starter code, and test feedback.
Prerequisites
- Python variables
- Lists or strings
- Basic for loop syntax
Difficulty and time
- Level
- Intermediate
- Estimated time
- 17 minutes
Practice path
Summary
Flatten only one level of nested lists, leaving deeper nested lists intact.
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
Mixed elements
Input
[1, [2, 3], 4]
Output
[1, 2, 3, 4]
Explanation
The inner list [2, 3] is flattened into the outer list. Other elements stay the same.
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 input 0
[[1,2],[3],[4,5]]
Sample output 0
[1, 2, 3, 4, 5]
Explanation 0
All inner lists are expanded into the returned list.
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