Unit tests for file_utils.py, demontrating mock_open.

file_utils_test.py

import unittest
import mock

import file_utils


class FileUtilsTest(unittest.TestCase):
    def test_read_file(self):
        with mock.patch("__builtin__.open", mock.mock_open(read_data="MOCKED"), create=True) as mock_file:
            result = file_utils.read_file("path")
        mock_file.assert_called_once_with("path", "r")
        assert result == "MOCKED"

    def test_write_file(self):
        with mock.patch("__builtin__.open", mock.mock_open(), create=True) as mock_file:
            file_utils.write_file("path", "content")

        mock_file.assert_called_once_with("path", "w")
        mock_file.return_value.write.assert_called_once_with("content")


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