Flatten an arbitrarily nested array

flatten.py

def flatten(items):
    """
    Flatten a nested list of integers.

    Passing a non-list instead returns a list containing the argument.
    """
    if not isinstance(items, list):
        return [items]

    result = []
    for i in items:
        for x in flatten(i):
            result.append(x)

    return result

# Use pytest to run this function.
def test_flatten():
    assert flatten(10) == [10]
    assert flatten([3]) == [3]
    assert flatten([4, 2, 5]) == [4, 2, 5]
    assert flatten([1, 2, [3, 4]]) == [1, 2, 3, 4]
    assert flatten([[1], [2, 4, [5, 2]], 31, [15, 2]]) == [1, 2, 4, 5, 2, 31, 15, 2]