def test_accepts_uppercase_letters(self): assert_equal(is_valid('AB'), True)

    @raises(InvalidInputError)
 def test_rejects_lowercase_letters(self): assert_equal(is_valid('Ab'), False)
 def test_rejects_numbers(self):           assert_equal(is_valid('A9'), False)
 def test_rejects_numbers(self):           assert_equal(is_valid('A9'), False)
 def test_accepts_uppercase_letters(self): assert_equal(is_valid('AB'), True)
 def test_rejects_empty_string(self):      assert_equal(is_valid(''),   False)
 def test_rejects_spaces(self):            assert_equal(is_valid('A '), False)