P03 (*) リストのK番目の要素を取得する

def main():
    print element_at([], 3) == None
    print element_at("a", 3) == None
    print element_at(["a"], 0) == None
    print element_at(["a"], 1) == "a"
    print element_at(["a"], 3) == None
    print element_at(["a", "b"], 0) == None
    print element_at(["a", "b"], 1) == "a"
    print element_at(["a", "b"], 2) == "b"
    print element_at(["a", "b"], 3) == None
    print element_at(["a", "b", "c", "d"], 0) == None
    print element_at(["a", "b", "c", "d"], 1) == "a"
    print element_at(["a", "b", "c", "d"], 2) == "b"
    print element_at(["a", "b", "c", "d"], 3) == "c"
    print element_at(["a", "b", "c", "d"], 4) == "d"
    print element_at(["a", "b", "c", "d"], 5) == None

def element_at(lista, n):
    if not lista or not isinstance(lista, list) or not n or len(lista) < n:
        return None
    return lista[n-1]

if __name__ == "__main__":
    main()