def digit_sum_exactly(n, k):
    pass

def test_digit_sum_exactly():
    # 5
    # 14
    digit_sum_exactly(20, 5)
    # 19
    # 28
    # 37
    # 46
    # 55
    # 64
    # 73
    # 82
    # 91
    digit_sum_exactly(100, 10)

test_digit_sum_exactly()


def power_table(n):
    pass

def test_power_table():
    # 1 1 1
    # 1 2 3
    # 1 4 9
    power_table(3)
    # 1 1 1 1 1
    # 1 2 3 4 5 
    # 1 4 9 16 25
    # 1 8 27 64 125
    # 1 16 81 256 625
    power_table(5)

test_power_table()


def get_summands(num_list, k):
    pass

def test_get_summands():
    assert get_summands([1, 2, 3], 5) == (2, 3)
    assert get_summands([1, 2, 3], 6) is None

test_get_summands()


def encrypt(text):
    pass

def test_encrypt():
    assert encrypt("pes nema kridla") == "sep amen aldirk"
    assert encrypt("Maly krok pro cloveka, velky skok pro lidstvo.") \
        == "ylaM kork orp ,akevolc yklev koks orp .ovtsdil"

test_encrypt()


def arrow(n, direction):
    pass

def test_arrow():
    # ..#..
    # .#...
    # #####
    # .#...
    # ..#..
    arrow(5, "left")
    # ...#...
    # ....#..
    # .....#.
    # #######
    # .....#.
    # ....#..
    # ...#...
    arrow(7, "right")


def remove_third_letter(words):
    pass

def test_remove_third_letter():
    words = ["apple", "banana", "xy", "abcdef", "a"]
    remove_third_letter(words)
    assert words == ['aple', 'baana', 'XXX', 'abdef', 'XXX']

    words = ["a", "ab", "abc", "abcd", "abcef"]
    remove_third_letter(words)
    assert words == ['XXX', 'XXX', 'ab', 'abd', 'abef']

test_remove_third_letter()