# Implement a prefix sum and a prefix list on arbitrary ‹Iterable›
# instances, using the iterator approach (class with an ‹__iter__›
# method).
#
# Examples:
#
#     dump( prefixes( [ 1, 2, 3 ] ) )   # [] [1] [1, 2] [1, 2, 3]
#     dump( prefix_sum( [ 1, 2, 3 ] ) ) # [ 1, 3, 6 ]

def prefixes( list_in ):
    pass

def prefix_sum( list_in ):
    pass


def test_main() -> None:
    import types

    pref = [ [], [ 1 ], [ 1, 2 ], [ 1, 2, 3 ], [ 1, 2, 3, 4 ] ]
    assert not isinstance( prefixes( [] ), types.GeneratorType )

    for i in prefixes( [ 1, 2, 3, 4 ] ):
        assert i in pref
        pref.remove( i )

    assert not pref

    lst = [ 1, 2, 3, 4, 5 ]
    out = [ 1, 3, 6, 10, 15 ]

    count = 0
    for item in prefix_sum( lst ):
        count += 1 # is iterable
    assert count == 5

    assert list( prefix_sum( lst ) ) == out


if __name__ == "__main__":
    test_main()