#0004

Flatten One Level of Nested Lists

EasyPython10 min10 XP

Problem

Given a list of lists, return a new list containing every inner item in left-to-right order.

Why This Matters

Flattening appears when APIs, grouped rows, or batched data return nested lists and you need a single sequence.

Function Signature

def flatten_once(groups):

Examples

Example 1

Inputgroups = [[1, 2], [], [3, 4]]

Output[1, 2, 3, 4]

The empty inner list contributes nothing, and the remaining values keep order.

Constraints

Only flatten one level.
Inner lists may be empty.
Do not mutate the input lists.

CodePython

def flatten_once(groups):
    """
    Input example: groups = [[1, 2], [], [3, 4]]
    Return: [1, 2, 3, 4]
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    result = []
    # write your code here
    return result

def flatten_once(groups):
    """
    Input example: groups = [[1, 2], [], [3, 4]]
    Return: [1, 2, 3, 4]
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    result = []
    # write your code here
    return result

Visible browser tests run here when available.

Testcases1 visible / 4 hidden categories

Includes empty group

Input[[1, 2], [], [3, 4]]

Expected[1, 2, 3, 4]

The nested loops visit inner lists in order.

Hidden Test Categories

All inner lists emptyStrings as itemsSingle inner listMany small groups