Recursion · routine

Flatten

Write flatten(items) for a list whose elements are numbers or more lists, nested any depth, returning one flat list of the numbers in order.

flatten([1, [2, [3, 4]], 5]) is [1, 2, 3, 4, 5]. isinstance(x, list) tells you whether x is a list.

right answers
arguments left as they should be

def flatten(items):
    result = []
    for x in items:
        if isinstance(x, list):
            for y in x:
                result.append(y)
        else:
            result.append(x)
    return result

Run adds print(flatten([1, [2, [3, 4]], 5])) after your code, to try it.

Stuck on the idea rather than the code? The Recursion lesson walks through it.