test.py

import unittest
import os


def custom_function(file_name):
    with open(file_name, 'rt') as f:
        return sum(1 for _ in f)


class CustomTests(unittest.TestCase):
    def setUp(self):
        self.file_name = 'test_file.txt'
        with open(self.file_name, 'wt') as f:
            f.write("""
                    파이썬에는 단위테스트 모듈이 기본으로 포함되어 있습니다.
                    매우 멋지군요!
                    단위테스트를 잘 수행해보고 싶습니다!
                    """.strip())

    def tearDown(self):
        try:
            os.remove(self.file_name)
        except:
            pass

    def test_run(self):
        custom_function(self.file_name)

    def test_line_count(self):
        self.assertEqual(custom_function(self.file_name), 3)

    def test_no_file(self):
        with self.assertRaises(IOError):
            # custom_function(self.file_name)
            custom_function('abc.txt')


if __name__ == '__main__':
    unittest.main()