from twttr import shorten


def main():
    test_uppercase()
    test_lowercase()
    test_null()
    test_digits()
    test_punctuation()


def test_uppercase():
    assert shorten('TWITTER') == 'TWTTR'


def test_lowercase():
    assert shorten('twitter') == 'twttr'


def test_null():
    assert shorten('') == ''


def test_digits():
    assert shorten('abc123') == 'bc123'


def test_punctuation():
    assert shorten('abc,') == 'bc,'


if __name__ == "__main__":
    main()