fsx is a low-level and easily mockable file system wrapper.

Your production code may write a file to disk.

create_file.py:

import fsx

def run():
    with fsx.open('/files/file1.txt', 'w') as file_:
        file_.write('Hello World')

You may write a test which checks if the content gets written correctly. Using the pytest-fixture fsx_fake you can create virtual files or directories. All calls to fsx will then work with the virtual instead of the real files.

test_main.py:

import fsx
from fsx.test import fsx_fake
import create_file

def test_hello_world_is_written(fsx_fake):
    fsx_fake.add('/files/')

    create_file.run()

    result = fsx.open('/files/file1.txt', 'r').read()
    assert result == 'Hello World'

fsx_fake will keep a tree of virtual files (and their contents) for the currently executing test.

pip install fsx

I think mocking built-in file system modules like os or glob directly is not a good idea. You may have 3rd party libraries which do require access to the real file system.

I do prefer to make a conscious effort and select which parts of my production-code are to be mockable.

If you just want to mock the Python's built-in file system modules you can use pyfakefs

• The fsx.glob module is slightly different from Python's built-in glob module. It ensures that calling glob/iglob will not swap forwardslashes for backslashes on Windows.