[Tutor] How do I test file operations (Such as opening, reading, writing, etc.)?

[boB Stepp]

I am starting a new project and wish to do it from beginning to end
using TDD best practices.  This is also my practical foray for the
first time into OOP.  The application will have a primitive editor to
create and edit certain text files that the program will need.  The
first file the program will need is one to be called classifiers.txt,
which will be a list of grammar abbreviations that will be used while
classifying sentences.  The beginning of such a file might look like:

N    # Noun
V    # Verb
ADJ  # Adjective

So I started out with the following test_main.py (Later I will factor
out code into separate modules once I know what will make sense.  For
now I am starting out with a single program file, main.py.):

#!/usr/bin/env python3

import unittest

import grammar.main

class TestEditor(unittest.TestCase):
    """
    Test all methods of the Editor class.
    """

    def setUp(self):
        self.editor = grammar.main.Editor()

    def test_edit_classifiers(self):
        # ???


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

And the current code in main.py:

#!/usr/bin/env python3

class Editor(object):
    def __init__(self):
        pass

    def edit_classifiers(self):
        pass

if __name__ == "__main__":
    main()

At this moment I am stumped as to what sort of test to write for the
method edit_classifiers().

At this very beginning stage, what I would want to have happen is to
call edit_classifiers, it will try to open classifiers.txt, realize
that it does not exist yet, and offer to create this file for the user
to start entering the needed grammar classifiers.

I don't want to mess with what will become the program's *real*
classifiers.txt (And other needed text files to come, that will
likewise be editable.), so how do I simulate these various needed file
operations in a way that tests the actual program code, but without
touching the actual data files?

I was looking at "How to Mock Python's `open` function" at
http://www.ichimonji10.name/blog/6/, which looks potentially
promising, but looks complicated.  This leads me to look at
https://docs.python.org/3.4/library/unittest.mock.html, but right now
I am finding this heavy going, and it will probably take me some time
and further research before I feel I am really understand applying
this mock process to file ops.  But is this direction I need to
investigate?

TIA!

-- 
boB