Flatten List

source.py

def flatten1(list_):
    '''Flatten nested list of integers into a single list.

    Args:
        list_ (list): list to be flattened

    Returns:
        Iterable[int]: the flattened list.
    '''
    result = []
    for item in list_:
        if isinstance(item, list):
            result = result + flatten1(item)
        else:
            result.append(item)
    return result


def flatten2(list_):
    '''Same as flatten1 but outputs as a generator in order to handle very
    large lists.

    Args:
        list_ (list): list to be flattened

    Returns:
        Generator[int, None, None]: the flattened list.
    '''
    for item in list_:
        if isinstance(item, list):
            yield from flatten2(item)
        else:
            yield item

test_flatten.py

# Uses PyTest Testing Format
from source import flatten1, flatten2


def test_flatten():
    testcase = [[1,2,[3]],4,[[5, 6], 7]]
    funcs = (flatten1, flatten2)
    for func in funcs:
        result = func(testcase)
        for item in result:
            assert not isinstance(item, list)