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Flatten a List of Lists

Beginner · Write the code · from 50 Python Practice Problems with Instant Answer Checking (Beginner to Advanced)

From the lesson: 50 Python Practice Problems with Instant Answer Checking (Beginner to Advanced)

Fifty problems, each with a code editor and real tests. Write your solution, press Check, and see exactly which cases pass. Stuck? Hints reveal one at a time, and the solution unlocks once you have checked at least once. Your code and progress are saved in this browser.

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Write flatten(rows) that turns a list of lists into one list, using a list comprehension.

flatten([[1, 2], [3], [], [4, 5]])  ->  [1, 2, 3, 4, 5]
def flatten(rows):
pass
Solution unlocks after your first Check

What Check runs

Your code runs first, then each of these lines. The output must match exactly.

RunsExpected output
flatten([[1, 2], [3], [], [4, 5]])[1, 2, 3, 4, 5]
flatten([])[]

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