test_mock_open.py

import os
import unittest
from unittest import mock


class TestMockOpen(unittest.TestCase):

    def test_mock_open(self):
        mock_open = mock.mock_open()
        with mock.patch('builtins.open', new_callable=mock_open()):
            with open('temp.file', 'w') as f:
                f.write('temp temp!')
        
            self.assertFalse(os.path.exists('temp.file'))
        self.assertFalse(os.path.exists('temp.file'))

    def test_create_file(self):
        with open('done.file', 'w') as f:
            f.write('done!')

        self.assertTrue((os.path.exists('done.file')))

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

$ ./python3 test_mock_open.py
..
----------------------------------------------------------------------
Ran 2 tests in 0.010s

OK