How to mock python build in methods

Question

I have a python method does the following:

1. list the files under a directory using os.listdir(/test)
2. regex match some of the files under the directory, put the files in a list
3. read the contents out of files in the list, do some aggregation stuff.

Obviously, the only interesting part for me to test in my case is 2, 3, so 1 is definitely something I want to mock against. I started doing patch file creation/deletion under /test folder in my setUp() and tearDown(). But colleague told me it's not good idea to do I/O in unitest.

so what's the best way to mock build in os.listdir() in my unitest? or what's the alternatives?

Is there anything I can do to achieve something like:

setUp() {
    #mock a few files eg.test1.txt, test2.txt, test3.txt under directory /test 
    #without physically creating them using I/O
}
tearDown() {
   #whatever cleanup required 
}

Answer 1

What about using the Mock module?

>>> import os
>>> from mock import MagicMock
>>> os.listdir = MagicMock(return_value=['file1.txt', 'file2.txt', 'file3.txt'])
>>> os.listdir('./test')
['file1.txt', 'file2.txt', 'file3.txt']

If you don't want to mokey-patch (ie. break) os, then you could use mock_os or the likes.

Read about starting and stopping:
http://docs.python.org/dev/py3k/library/unittest.mock.html#patch-methods-start-and-stop

And:
http://docs.python.org/dev/py3k/library/unittest.mock.html#quick-guide