Creating The Readable Mock

To mock a file you can use StringIO and an input string. Since the StringIO will only traverse once (like a regular file) it might be convenient to make a mock builder.

def mock_open(data_string):
    mock_file = MagicMock(spec=file)
    mock_file.return_value = StringIO(data_string)
    return mock_file

Using the Mock

To use the mock you patch the built-in open function.

opened = mock_file()

with patch('__builtin__.open', opened, create=True):
    run_tests_here()

Checking the Mock

To see if the file was opened correctly:

opened.assert_called_with(filename, r_or_w)

To check write calls look at opened.write.mock_calls.